CAN's scratch command should push existing scratchfile content below the fold

Running CAN's scratch command while a scratchfile exists should push all existing content beneath a new fold. This is done by adding --- to the top of the scratchfile.

I'll do some research into a simple interface for prepending to a file on disk. I can always use the naieve way of reading the whole file and sticking the string we want onto the start for now if nothing useful pops up, and think about the problem more deeply if/when the naieve way causes problems. The long-and-short of it is that there is no simple way to prepend to a file on disk.

As the scratchfile should not get huge, the naieve approach is acceptable here until it isn't any more.

The scratch command should fire COMMAND_SCRATCH (issue #32, note). The beneath-the-fold handler should listen for this event and perform the operation.

I should make sure that the manager notices this change. It definitely should, just being paranoid.