Add keyfreq-reset command.

keyfreq-reset command clears all the stats so you start a fresh. It
deletes the file and clears the hash without any backup so BE CAREFUL.

Possible improvement which would make sense is to backup the old file first, maybe optionally.
I don't know if this feature makes sense to somebody else but I sometimes measure just some parts of the work and like to start from scratch and this command is a convenience.