Support ~/.ssh/config for SSH tab completion

[EspadaV8]

I would do this as a pull request but I'm unable to only submit the single change without 2 random commits that aren't needed.

The commit can be seen here - EspadaV8@42b5a1d

It adds a simple parse of the ~/.ssh/config file and adds them to the tab completion for SSH. I'm sure that the code can be cleaned up since I hacked it together. The use of 'grep' isn't very nice but I'm unsure how this should be done as part of ZSH.

[robbyrussell]

Closing this per #2568.