Add support for syntax highlighting more languages

[ankhers]

I was hoping that a simple

```elixir my_code_here() ```

would support syntax highlighting in the generated presentation. Unfortunately that does not seem to be the case. I would love to see this added to slipshow.

I would also be happy to submit a PR for this feature if you would be willing to point me in the right direction. Is this simply a theming issue or is this something that the engine does not yet support?