Resolve SSH host aliases by reading from `~/.ssh/config`

This allows someone to have something like this in their SSH config:

    Host gh
      HostName github.com

And then use git remote URLs such as:

    gh:myname/myrepo.git

Since git itself allows people to do this, we need to support it in hub.
Normalize "ssh.github.com" hostname to just "github.com"