Lazy load markup gems

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