Adding config options for ignoring hosts in SSH config by name or keyword

This adds two options to the Shuttle config to deal with ignoring entries in the user's SSH config without having to ignore the entire file.  

The first option is `ssh_config_ignore_hosts` which is a list of strings.  If a host name matches any of these strings exactly, it will not be included in Shuttle's server list.  `server.example.com` only matches `server.example.com`.

The second is `ssh_config_ignore_keywords` which is another list of strings.  If a host name contains any of these strings as a substring, it will not be included in Shuttle's server list.  `example` matches `server.example.com` and `example.something.com`.

This is my first attempt at Objective-C, so don't feel bad telling me if there is a better way to do something.

Fixes #63