Syntastic feature parity (thereabouts)

This issue will serve as a meta issue. I'm aware there are a bunch of users who came to ALE from Synastic, which is no longer maintained. I'm interested in knowing what little things there are that we can tweak to support some things in Synastic that ALE still doesn't have.

We will not blindly do everything Syntastic did. There will be some differences, and for good reason. There will be some things we do differently in general. We should also list here the things we're not going to do for future reference.