feat(filters): limit the number of posts

[shouya]

This PR adds a new filter limit that allows limiting the number of posts. This filter operates in two modes:

• count mode: only the first n posts are kept
• duration mode: only posts published within duration are kept

The format of duration string follows the same format as duration strings used in other places. Find more information about the duration format at duration_str.

The config syntax as follows:

  - path: /hackernews-fresh.xml
    source: https://news.ycombinator.com/rss
    filters:
      - limit: 8h

  - path: /hackernews-first-10.xml
    source: https://news.ycombinator.com/rss
    filters:
      - limit: 10

Fixes #88.