Move some lexers to a separate rouge/contrib gem.

[jneen]

Despite Rouge being eminently extensible in terms of new lexers (I have several projects that just subclass Rouge::Lexer directly) there seems to be a broad misunderstanding that languages must be in Rouge core in order to be used.

That being said, projects like Gitlab do need to support some flavour of "all languages", so it is often useful to have some amount of centralization. I think an external contrib library is a nice balance that a) shows that you can extend Rouge without too much fuss, and b) provides a more appropriate place for smaller, niche languages to live.

@tancnle thoughts?

[tancnle]

@jneen This is great idea and I am supportive. I like the approach used by highlight.js. They only supports 40 core languages while the rest are third-party plugins.

[jneen]

I suppose then the difficult decision is which languages are our core. Right now rouge has 200+ lexers, which is a bit of a beast to deal with.