feat(filters): new merge_feed filter

[shouya]

Implements #11.

The new filter's name is merge_feed and the config type is either:

• an string, which is interpreted as the url of the source to merge
• or a full config object with the following properties:
  • source (string, required): the source url of the extra feed
  • client (string, optional): any Client config
  • filters (string, optional): a list of filters to run on the extra feed before merging it

Here's an example config using the merge_feed filter:

endpoints:
  - path: "/my-feed"
    source: "https://example.com/feed"
    filters:
      - merge_feed: "https://example.com/extra-feed"

  - path: "/my-feed"
    source: "https://example.com/feed"
    filters:
      - merge_feed:
          source: "https://example.com/extra-feed"
          client:
            timeout: 10
          filters:
            - full_text: {}
            - js: |
              function modify_post(feed, post) {
                post.title += " (merged);
                return post;
              }