Add glob negation support

[andreynering]

Hi @mattn, thanks a lot for maintaining this lib! I've being using it for a long time and it's pretty stable and fast!

Negation would allow to exclude a given patten from the returned files.

For example, this would return all .go files, but not those living in the vendor/ directory:

./!(vendor)/**/*.go

[mattn]

Fixed in last change.

[marco-m]

thanks! :-)