Support negation of pattern

[torkel-hoge]

This PR adds new syntax which allows to negate the pattern or its part. It extends current AnyOf's syntax with ! char. For example, {!foo,bar} pattern will match baz string and will not match foo and bar. See more complicated examples in glob_test.go.

This seems to resolve the #47.

Feel free to request changes in the code and more test cases.

[torkel-hoge]

@gobwas hey, is there any chance that this change will be accepted or at least reviewed?
I see that the project is not actively maintained, but still it's actively used by our project Runtime Radar and we'd be glad if the new syntax is added.