Remove flipNegate from minimatch options.

This tries to address: https://github.com/atom/find-and-replace/issues/149

From what I can see the only thing stopping people from using ```!**.css``` in file searching on atom is that negation is explicitly turned off.

Couldn't really find a reason why it's turned off at the moment? (Maybe you can help @benogle ?)

Closes atom/find-and-replace#149