notify-status: Add notify-status plugin

notify-status: Add notify-status plugin

This enables updating a dictionary with mailbox
status information when the mailbox changes.

It requires notify_status_dict setting for specifying
the dict where the data is stored.

One can optionally use notify_status_mailbox patterns
for specifying which box(es) the status update is done.
Subsequent patterns are notify_status_mailbox2 and so forth.

One can also specify data format using notify_status_value
plugin setting.