Added the ability to specify a custom jshint module

I'm currently in need of a jshint feature that isn't supported in the version bundled with SublimeLinter. It seems like the versions are somewhat locked, and not customizable so I hacked support (for jshint only) into the plugin.

A "bug" in this code is that the setting is only being read from the global settings for some reason. I didn't figure out how SublimeLinter was accessing it's own setting files.

I have never worked in the sublime plugin environment (or really python for that matter) so if you have the time please let me know how I can fix the mistakes I've made.

Thanks!