Add a negate (!) prefix to Glob matchers

[bep]

I'm planning to make roles and versions slices of Glob patterns. Other places in Hugo we have includes/excludes pair of slices for this, which gives us great control over what to include, but it makes for a verbose setup having to have 2 settings for one thing.

So instead, I suggest we allow a ! followed by a single space, e.g.

versions = ["! v2*"]

Would exclude all v2* versions.