Allow custom paths to be watched when using regolith watch

[miguelkjesus]

Currently regolith watch only watches files within the BP/, RP/ and data/ folders. It would be useful to add an option to supply extra paths that should be watched.

My use case is that Im developing a library, and Id like regolith to rebuild whenever a package in node_modules changes. Right now, this isn't possible since regolith watch doesn't track changes outside its default scope.