Ignore replacing ssh host that fixes limited network

ssh.github.com is a hack for people on limited networks to be able to use SSH through HTTPS port. We should ignore replacing it when constructing URLs.

As part of the fix, refactor into SSHConfig and SSHConfigReader to make code testable. It also fixes a bug on `hostReStr` that doesn’t match the “Hostname” string in ssh config.

Issue: https://github.com/github/hub/issues/672
Merge pull request #679 from github/ssh_url_fix

Ignore replacing ssh host that fixes limited network