I added a config option to show attribute names by errors

Hey dude,

This is a follow up to my issue post: https://github.com/justinfrench/formtastic/issues/919

If this gets accepted I'll include the functionality in formtastic-bootstrap ASAP and put in another pull request for that. I've tried my best to do this touching as little code as possible. This is my first pull request so any feedback you could offer would be much appreciated. :-)

Cheers,

Mike