Start putting commands in command/, modify core
command: move more to this package, remove old packages
command: move all remaining commands
packer: remove Command
packer/rpc, packer/plugin: remove Command interfaces
Rename some files, style
Remove version from "packer" package
command: support machine-readable still
command: version tests
remove command configs
Merge pull request #1629 from mitchellh/f-no-command-plugins

Remove command plugins, command interface from core, etc.
Merge branch '0-8-0'
update CHANGELOG