Add a flag to prevent from fields reorder

[subchen]

Current the fields of output struct is sorted, it is not good for me to advanced process.
If the fields order is same with original json format, it is more easy to use for me.

[ChimeraCoder]

Could you elaborate on the use case for preserving the order? Field names aren't quite like imports, but in Go, it's generally preferred to keep things in alphabetical order.

[subchen]

For example: I got a json from https://api.github.com/repos/ChimeraCoder/gojson

{
    "id": 7344683,
    "name": "gojson",
    "full_name": "ChimeraCoder/gojson",
    "owner": {
        "login": "ChimeraCoder",
        "id": 376414,
        "avatar_url": "https://avatars0.githubusercontent.com/u/376414?v=4",
        "gravatar_id": "",
        "url": "https://api.github.com/users/ChimeraCoder",
        "html_url": "https://github.com/ChimeraCoder",
        "followers_url": "https://api.github.com/users/ChimeraCoder/followers",
        "following_url": "https://api.github.com/users/ChimeraCoder/following{/other_user}",
        "gists_url": "https://api.github.com/users/ChimeraCoder/gists{/gist_id}",
        "starred_url": "https://api.github.com/users/ChimeraCoder/starred{/owner}{/repo}",
        "subscriptions_url": "https://api.github.com/users/ChimeraCoder/subscriptions",
        "organizations_url": "https://api.github.com/users/ChimeraCoder/orgs",
        "repos_url": "https://api.github.com/users/ChimeraCoder/repos",
        "events_url": "https://api.github.com/users/ChimeraCoder/events{/privacy}",
        "received_events_url": "https://api.github.com/users/ChimeraCoder/received_events",
        "type": "User",
        "site_admin": false
    },
    "private": false,
    "html_url": "https://github.com/ChimeraCoder/gojson",
    "description": "Automatically generate Go (golang) struct definitions from example JSON",
    "fork": false,
    "url": "https://api.github.com/repos/ChimeraCoder/gojson",
    "forks_url": "https://api.github.com/repos/ChimeraCoder/gojson/forks",
    "keys_url": "https://api.github.com/repos/ChimeraCoder/gojson/keys{/key_id}",
    "collaborators_url": "https://api.github.com/repos/ChimeraCoder/gojson/collaborators{/collaborator}",
    "teams_url": "https://api.github.com/repos/ChimeraCoder/gojson/teams",
    "hooks_url": "https://api.github.com/repos/ChimeraCoder/gojson/hooks",
    "issue_events_url": "https://api.github.com/repos/ChimeraCoder/gojson/issues/events{/number}",
    "events_url": "https://api.github.com/repos/ChimeraCoder/gojson/events",
    "assignees_url": "https://api.github.com/repos/ChimeraCoder/gojson/assignees{/user}",
    "branches_url": "https://api.github.com/repos/ChimeraCoder/gojson/branches{/branch}",
    "tags_url": "https://api.github.com/repos/ChimeraCoder/gojson/tags",
    "blobs_url": "https://api.github.com/repos/ChimeraCoder/gojson/git/blobs{/sha}",
    "git_tags_url": "https://api.github.com/repos/ChimeraCoder/gojson/git/tags{/sha}",
    "git_refs_url": "https://api.github.com/repos/ChimeraCoder/gojson/git/refs{/sha}",
    "trees_url": "https://api.github.com/repos/ChimeraCoder/gojson/git/trees{/sha}",
    "statuses_url": "https://api.github.com/repos/ChimeraCoder/gojson/statuses/{sha}",
    "languages_url": "https://api.github.com/repos/ChimeraCoder/gojson/languages",
    "stargazers_url": "https://api.github.com/repos/ChimeraCoder/gojson/stargazers",
    "contributors_url": "https://api.github.com/repos/ChimeraCoder/gojson/contributors",
    "subscribers_url": "https://api.github.com/repos/ChimeraCoder/gojson/subscribers",
    "subscription_url": "https://api.github.com/repos/ChimeraCoder/gojson/subscription",
    "commits_url": "https://api.github.com/repos/ChimeraCoder/gojson/commits{/sha}",
    "git_commits_url": "https://api.github.com/repos/ChimeraCoder/gojson/git/commits{/sha}",
    "comments_url": "https://api.github.com/repos/ChimeraCoder/gojson/comments{/number}",
    "issue_comment_url": "https://api.github.com/repos/ChimeraCoder/gojson/issues/comments{/number}",
    "contents_url": "https://api.github.com/repos/ChimeraCoder/gojson/contents/{+path}",
    "compare_url": "https://api.github.com/repos/ChimeraCoder/gojson/compare/{base}...{head}",
    "merges_url": "https://api.github.com/repos/ChimeraCoder/gojson/merges",
    "archive_url": "https://api.github.com/repos/ChimeraCoder/gojson/{archive_format}{/ref}",
    "downloads_url": "https://api.github.com/repos/ChimeraCoder/gojson/downloads",
    "issues_url": "https://api.github.com/repos/ChimeraCoder/gojson/issues{/number}",
    "pulls_url": "https://api.github.com/repos/ChimeraCoder/gojson/pulls{/number}",
    "milestones_url": "https://api.github.com/repos/ChimeraCoder/gojson/milestones{/number}",
    "notifications_url": "https://api.github.com/repos/ChimeraCoder/gojson/notifications{?since,all,participating}",
    "labels_url": "https://api.github.com/repos/ChimeraCoder/gojson/labels{/name}",
    "releases_url": "https://api.github.com/repos/ChimeraCoder/gojson/releases{/id}",
    "deployments_url": "https://api.github.com/repos/ChimeraCoder/gojson/deployments",
    "created_at": "2012-12-27T19:10:50Z",
    "updated_at": "2018-02-04T16:46:09Z",
    "pushed_at": "2018-01-30T05:56:31Z",
    "git_url": "git://github.com/ChimeraCoder/gojson.git",
    "ssh_url": "git@github.com:ChimeraCoder/gojson.git",
    "clone_url": "https://github.com/ChimeraCoder/gojson.git",
    "svn_url": "https://github.com/ChimeraCoder/gojson",
    "homepage": "",
    "size": 209,
    "stargazers_count": 1581,
    "watchers_count": 1581,
    "language": "Go",
    "has_issues": true,
    "has_projects": true,
    "has_downloads": true,
    "has_wiki": true,
    "has_pages": false,
    "forks_count": 100,
    "mirror_url": null,
    "archived": false,
    "open_issues_count": 31,
    "license": {
        "key": "gpl-3.0",
        "name": "GNU General Public License v3.0",
        "spdx_id": "GPL-3.0",
        "url": "https://api.github.com/licenses/gpl-3.0"
    },
    "forks": 100,
    "open_issues": 31,
    "watchers": 1581,
    "default_branch": "master",
    "network_count": 100,
    "subscribers_count": 40
}

This json is bigger. It is also not in alphabetical order.

I can have a good view for field priority and similar fields in group.

I think the order in original json is valuable for us, we should follow them.