Add extra watch folders

[daveaglick]

With processes we might want to watch a folder that isn't actually part of the input folders. This would let the execution loop along with processes fire when a file in one of the watched folders changes but not the input folder(s).

For example, a gulp task that operates on assets not in one of the watched folders that isn't a background process. We want gulp to kick off and update the output folder when a file in it's assets folder changes, but we don't want that folder to be an input folder.

Note that while we might not necessarily want to also kick off another Statiq execution when the assets folder changes, that seems like an acceptable behavior and working around it would be tricky.

[daveaglick]

Added a WatchPaths setting key to add additional watch folders, will ship with next release.