add config option in watcher to ignore directories named something

[wcpettus]

When changing hornet configs (e.g. streaming vs triggered delay times) we frequently may forget to purge the directory structure, resulting in a useless hornet.

This PR:

• causes hornet to recurse the watcher directory(ies) and pickup all existing subdirectories to watch
• adds an option to ignore a directory name (e.g. "lost+found")

[nsoblath]

The code updates look good. I have a minor request: please add a directory-ignoring setting to the example config (in the examples directory).