Abstract CommandBarHistory to new Readline History class

[thehunmonkgroup]

#212 will add the rest of the Readline shortcuts, and abstract them to a Readline class.

One thing left over from this move is that the history management still lives in command_bar.py under a CommandBarHistory class. The class is pretty generic -- add, previous, next, cleanup, across multiple commands.

So CommandBarHistory should be moved into Readline as a generic History class, probably changing the command arguments to something more generic like tag or type.