CAN's scratch command should fire the COMMAND_SCRATCH event

When a CAN user sends a scratch command, the COMMAND_SCRATCH event should be fired to allow handlers to respond. This should include whether the scratchfile exists as a boolean argument.

Backlinks

• CAN's scratch command should push existing scratchfile content below the fold
  • The scratch command should fire COMMAND_SCRATCH (issue #32, note). The beneath-the-fold handler should listen for this event and perform the operation.