Remove command plugins, command interface from core, etc.

This PR starts the core changes on 0.8.0 by removing the concept of a command plugin and even removes the notion of a command from the `packer` package. This is in an effort to move towards a more simplified core for Packer.

As an additional benefit, we were able to switch to the mitchellh/cli lib that we use in all our other projects.

Command plugins were something we never saw widely used, and don't seem altogether useful.