Lazy load markup gems

[haileys]

github-markup currently eager loads markup gems when it's required. While this only takes about 100ms when testing the gem in isolation, when it's incorporated into a larger application like the GitHub rails app with a massive $LOAD_PATH, the time taken to load github-markup balloons out to 250-300ms.

This pull request adds lazy loading to github-markup so that markup gems are only required the first time they're used. For production environments where everything should be loaded up front, I've added a GitHub::Markup.preload! method that eager loads all markup gems straight away.

cc @github/perf

[haileys]

Ended up doing a bit of a refactoring heh. cc @github/markup for review

[haileys]

Also cc @defunkt and @gjtorikian since you two are the top two contributors

[gjtorikian]

Wow, good stuff. 💖

Not even going to pretend I understand all the perf improvements, but I say . I actually don't have publishing rights for this gem yet so I can't cut a new release, sadly. Seems it should warrant a minor version bump though.

[haileys]

@gjtorikian I think this deserves a major version bump (1.0.0 maybe?) because it does have breaking API changes.

Actually with that in mind, I might go and clean up a bit more cruft.

[haileys]

@gjtorikian Also the perf improvement is basically just not requiring gems until they're actually needed in development. In production, we'll still require them all up front.