Support negative globs in the `watch` option

When the `watch` option is an array, it tells the generator to rerun when a matching path within the root directory is touched. By including a negative glob, the caller can effectively tell the generator “but don't automatically watch *these* files”. Specifically, it's preventing files that match the normal globs in the array from being watched if they also match a negated glob in the array. It does *not* affect other file operations performed by the generator.