language feature usage statistics

[Adam D. Ruppe]

I've actually considered doing this with dpldocs.info before. It 
would only hit public code but... I have copies of basically the 
whole dub repo and code that is already custom parsing it so I 
could possibly pull info like this when it does its updates.

Though my parser doesn't always keep up with new features (I 
often skip function bodies since it isn't super important for 
documentation purposes) it still mostly works.