feat(filters): add modify_post and modify_feed filters

[shouya]

Fixes #38.

The config syntax looks like:

filters:
  - modify_post: |
        if (feed.title === 'Hacker News') {
          post.title = post.title.replace('Show HN: ', '');
        }
  - modify_feed: feed.title = 'HN';

This implementation is quick and dirty as it merely template the code in modify_post and modify_feed like this:

function modify_post(feed, post) {
  <your code>;
  return post;
}