feat(filters): add convert_to filter for converting feed format

[shouya]

This filter converts the feed into the specified format. It can be used to convert an Atom feed into an RSS feed, or vice versa. It can be useful for merge to merge feeds of different formats.

Usage:

- path: /feeds
  source: https://example.com/atom.xml
  filters:
    - convert_to: rss # or atom

Fixes #42.