It would be nice to have an API like:

GitHub::Markup.language("README.md")
# => "markdown", or GitHub::Markup::Lanuage::MARKDOWN, or whatever

This would be useful for including in cache keys on github.com so we can make sure to re-render a file whenever its language changes. Right now we have to use the file's name instead as a proxy for this. But if something changes in GitHub::Markup's language-detection logic the cache won't get busted. Including the actual detected language in the cache key would fix this.