Babel caching

[sindresorhus]

Should be done after #117.

See: #103 (comment)

Compilation is a lot larger overhead than just reading in a cached file from disk. I do think we can do caching better than Babel itself, though. Mainly using separate cache files for each test file (Babel uses one large file) and a LRU cache.

[sebmck]

Would be happy to have this in Babel core. Everything is fairly modularised internally so the only files you should need to touch are here and here.

[sindresorhus]

@sebmck Oh cool. Are you ok with exposing the cache functionality? We're not using the require hook, so we need to be able to access the cache functionality from the Babel API.

[jamestalmage]

Caching is in master. PR #352