Allow excluding files from the `setup()` function

[norcalli]

Possible syntax choices here are:

• Using ! such as setup({'*',!vim})
• Passing an option such as setup {'*', vim = {exclude = true}}
• Both of the above.

I'm leaning towards the first one as long as people realize that !* would make little to no sense. The utility of an option would only be in the case of configuring it dynamically, which I don't think people would do.