Feature request - filter paths in :recursive mode

[sebbASF]

When monitoring a large directory tree, a lot of system resources can be required.
If one is only interested in a small subset of the tree, it would be useful to be able to ignore unwanted subtrees.

One way to do this might be to provide a filter to the INotify::Notifier#watch method, perhaps via an additional flag.
For maximum flexibility, the filter would be a boolean function, but a list of regexes might suffice.