_command_handler needs to be extensible about knack

[derekbekoe]

It's tricky finding a balance between making everything extensible and suggesting to subclass. In this case, I'd subclasses would be required anyway so you could subclass and provide your own implementation of create_command and then _command_handler.

In Azure CLI case, even if _command_handler was extensible, you'd still have to override create_command for this.

This is related to #12 but in that case, it was a single line you wanted to change to use a different class. Here, modifying _command_handler basically changes all of create_command anyway.

link to code in question: https://github.com/Microsoft/knack/blob/master/knack/commands.py#L106