Add 2 User events when mapping and unmapping

delimitMate has some nice logic for doing per-filetype mappings, but
this logic can't be used when users provide mappings directly. To remedy
this, add two User events `delimitMate_map` and `delimitMate_unmap` that
are fired at the appropriate times. Users can use these events to define
per-filetype mappings just as delimitMate does.