switch syntax highlighting to Linguist w/ auto-detection

Auto-detection is on by default, but can be turned off via a context
option. Note that auto-detection is happy to classify even a simple run
of text as a programming language:

  <pre>This is my little poem</pre>

We might consider turning it off by default to keep backwards
compatibility and reduce surprising behavior. With fenced code blocks in
GitHub Markdown, it's easy to tag code blocks with a "lang" attribute.