Add support for the Rouge syntax highlighter

[mojavelinux]

Allow the use of the Rouge syntax highlighter by setting the value of the source-highlighter attribute to rouge.

[mojavelinux]

Before merging, I'd like to implement a Prawn formatter that creates text fragments directly to avoid having to parse the generated HTML. This is similar to what we've done for CodeRay.

[mojavelinux]

And once that is done, I think we should just drop support for Coderay and map the "coderay" source highlighter value to Rouge.

[mojavelinux]

Prawn formatter implemented. I also implemented support for line numbers (with primitive, but sufficient, output).

[mojavelinux]

All that's really left to do is allow the start number to be customized when line numbers are enabled.

[mojavelinux]

We have to add a trailing endline or else certain highlighting rules don't take affect on the last line. (See rouge-ruby/rouge#279). Although this endline is benign in Prawn, we could strip it out before applying the formatting using the following code:

tokens = tokens.entries
if (last_token = tokens_a[-1])
  if (last_val = last_token[-1]) == EOL
    tokens.pop
  else
    last_token[-1] = last_val.chomp
  end
end

[rlopez133]

👍 on Rogue highlighter

[mojavelinux]

I'd like to do some side-by-side comparison of real-world code snippets to see how good a job Rouge does vs CodeRay and Pygments. I've got a bunch of repositories cloned, so I'll do a quick scan and see what comes up. If you're following this issue, free to test on any snippets you have to see if any deficiency come up in Rouge.