allow exclude pattern in watch

[jellevandenhooff]

Because of some silly reasons, I want to watch all go files, except those in a certain directory. Maybe a watch = ["**/*.go", "!foo/**/*.go"] could do the trick?

[amonks]

another great idea from @jellevandenhooff!

same status as #131: I'd like to make this change, but it won't happen for at least a week or so.