Turn unknown fields in `hie.yaml` into warnings

[fendor]

Until now, we threw an error message. The point being that you notice you didn't write the correct configuration and something is not right.

This will make it hard to introduce new optional keys, because older HLS versions will now choke on that config.

I think a warning is better. cabal-install also does this:

$ cabal build
Warning: /home/hasufell/git/stack2cabal/cabal.project: Unrecognized field
'foo' on line 11

Originally posted by @hasufell in #329 (comment)

[fendor]

Seems like yaml and aeson can't really report unknown fields as warnings T_T
haskell/aeson#808