Better markdown support and basic syntax highlighting; see #68

[phoboslab]

This improves markdown parsing:

• _emphasis_ in code blocks (such as in some_test_function()) will be left alone
• URLs are automatically recognized
• some basic syntax highlighting for a number of languages

The syntax highlighting is somewhat quick-and-dirty, but I didn't want to import a whole other library. In my tests it works quite well.

[jart]

Thanks so much for taking the time to help us solve #68. I like it! Reading through your code makes me feel excited to add additional definitions for my favorite languages.