feat: create feed from scratch

[shouya]

Fixes #35.

Now it's possible to specify in the source field (under endpoint, or in merge filter's config) a struct that looks like this:

- path: /feed.xml
  source:
    format: rss # or atom
    title: "My Feed"
    description: "This is my feed"
    link: "https://example.com/"

Don't forget to ensure the inspector ui has proper support for it before merging.