Read hosts from ssh_config

[crazyscot]

This will resolve #22 and should appear in the 0.2 release.

ssh_config features supported:

• One host with multiple aliases (e.g. Host foo bar)
• Wildcard matches

ssh_config features not currently supported (largely because the ssh2_config crate does not support them):

• Include directives
• Match patterns
• CanonicalizeHostname and friends