Support negative globs in the `watch` option

[aleclarson]

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.