Add author and created_by data

[jonjohnsonjr]

Fixes #42

We might not want to embed the import path, since that isn't really content-addressable, but we might already be embedding that somewhere? Not sure...

$ crane config $(ko publish github.com/google/ko/cmd/ko 2>/dev/null) | jq .
{
  "architecture": "amd64",
  "author": "github.com/google/ko",
  "created": "1970-01-01T00:00:00Z",
  "history": [
    {
      "author": "Bazel",
      "created": "1970-01-01T00:00:00Z",
      "created_by": "bazel build ..."
    },
    {
      "author": "ko",
      "created": "0001-01-01T00:00:00Z",
      "created_by": "ko publish github.com/google/ko/cmd/ko",
      "comment": "kodata contents, at $KO_DATA_PATH"
    },
    {
      "author": "ko",
      "created": "0001-01-01T00:00:00Z",
      "created_by": "ko publish github.com/google/ko/cmd/ko",
      "comment": "go build output, at /ko-app/ko"
    }
  ],
  "os": "linux",
  "rootfs": {
    "type": "layers",
    "diff_ids": [
      "sha256:01092e5921c5543a918d54d9df752ee09a84c912a1d914b7eb37e7152f20b951",
      "sha256:8f8d58d9708807553434fbd82014ea40b5ee0dc8b2eeefcde561a363461d3e66",
      "sha256:6abda58a9e4921b307ea22d1d404b97f0daad8965a5e9e64adc8228387f3c46c"
    ]
  },
  "config": {
    "AttachStderr": false,
    "AttachStdin": false,
    "AttachStdout": false,
    "Cmd": null,
    "Healthcheck": null,
    "Domainname": "",
    "Entrypoint": [
      "/ko-app/ko"
    ],
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
      "SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt",
      "KO_DATA_PATH=/var/run/ko"
    ],
    "Hostname": "",
    "Image": "",
    "Labels": null,
    "OnBuild": null,
    "OpenStdin": false,
    "StdinOnce": false,
    "Tty": false,
    "User": "",
    "Volumes": null,
    "WorkingDir": "",
    "ExposedPorts": null,
    "ArgsEscaped": false,
    "NetworkDisabled": false,
    "MacAddress": "",
    "StopSignal": "",
    "Shell": null
  },
  "container_config": {
    "AttachStderr": false,
    "AttachStdin": false,
    "AttachStdout": false,
    "Cmd": null,
    "Healthcheck": null,
    "Domainname": "",
    "Entrypoint": [
      "/ko-app/ko"
    ],
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
      "SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt",
      "KO_DATA_PATH=/var/run/ko"
    ],
    "Hostname": "",
    "Image": "",
    "Labels": null,
    "OnBuild": null,
    "OpenStdin": false,
    "StdinOnce": false,
    "Tty": false,
    "User": "",
    "Volumes": null,
    "WorkingDir": "",
    "ExposedPorts": null,
    "ArgsEscaped": false,
    "NetworkDisabled": false,
    "MacAddress": "",
    "StopSignal": "",
    "Shell": null
  }
}