Require subparser

There is a subtle bug in the way cleanslate was set up which caused every cleaner to automatically run on each call. This isn't so good once we have multiple cleaners. To remedy this problem a little extra work is put into setting up cleaner classes (setting a variable) which will create a sub_parser that triggers the cleaner.

That means, to call a cleaner that takes no arguments you would still need to specify its name as an argument:
`cleanslate.py some_cleaner`