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. 

This also helps our release size by cutting out 4 binaries.