Allow workouts to be cleaned

[tom-dudley]

It would be useful to be able to clean workouts which have been added. I added the example 80k plan but couldn't find a good way to remove all the workouts without doing it manually.

[mgifos]

This is what I meant: 1fbaa3c

We already have delete option -x, so I just wanted to have import/schedule as optional commands. This is according to the existing tool usage:
Usage: quick-plan [import|schedule] [options]
as -x is a global option that can be used in combination with any command.