implement liquid blocks for blockquote, pullquote, twitter, img and
youtube
test liquid block detection
document liquid plugins
Merge branch 'feature/liquid-filter', closes #115

Currently, the following tags are ported:

- blockquote -- generate beautiful, semantic block quotes
- img -- easily post images with class names and titles
- youtube_ -- easy embedding of YouTube videos
- pullquote -- generate CSS only pull quotes — no duplicate data, no javascript
- tweet -- embed tweets using Twitter's oEmbed API