Allow the type checker to target a specific folder

When using the Hack type checker, the `.hhconfig` must be present at the root of the source directory, which will type check the entire folder. However, this causes all errors from all code to be shown (which is OK), but can be very distracting when you are trying to fix type issues in a specific folder.

Would it be possible to mute all errors besides a target folder? Without breaking the relations/lookup from the other folders? Something like `hh_client --filter=path/to/folder`?

A guide on all the `hh_client` options would be nice too :P