Print failed snippet

If a test fails, print 3 lines of the snippet context underneath the failure.

It looks something like this:
![screen shot 2014-05-30 at 4 00 24 pm](https://cloud.githubusercontent.com/assets/627285/3137279/ee9c080e-e84f-11e3-9837-0c942d50a8fc.png)

Done in the PR:
- extract snippet logic from `html.rb` to `snippet.rb`
- make snippet 3 lines instead of 1 line
- add snippets to abstract formatter