Optionally follow symlinks within a watch root

[smashwilson]

Teach watchPath() a followSymlinks option. When true, any directories symlinked within a watched root directory should also be watched for changes. Newly created symlinks should be watched and removed symlinks should be unwatched as appropriate.

Note that this is a bit involved because none of the OS-level event APIs actually support following symlinks natively. This means that we'd need to do a full recursive scan of the watch root on root addition to discover symlinks, which means race conditions related to symlinks created or modified during the scan...