src/plugin.[ch]: Add `notification meta data' to notification_t.

src/plugin.[ch]: Add `notification meta data' to notification_t.

The plan is to have arbitrary values attached to notifications that user
defined actions can use to get machine parseable values for building
own notification strings or the like.

For example, the threshold checking stuff will (in the near future) add
the current (offending) value as well as the threshold values themselves
to the notification, so that notifying plugins don't need to parse the
(freely formatted) notification string to get that information.