Support Hiccup syntax sugar for multiple classes. Fixes #2.

Hiccup lets you write `[:div.foo.bar]` to denote `<div class="foo bar"></div>`. This patch adds support for this shorthand, taken directly from [the Hiccup compiler](https://github.com/weavejester/hiccup/blob/master/src/hiccup/compiler.clj#L62).