Introduce a structure for plugins and themes to be packaged

Further documentation will come in the README to explain this
structure. Plugins and themes included in these directories will
be copied with the Grunt process.
Public and private directories should be created manually

This should be on the maintainer of the specific installation as
these plugins could be coming in through any number of build
processes. Ours - git - would start to confuse things here.
Copy public build plugins to wp-content build first
And then copy over any plugins in the private directory
Remove private and public theme directories
Update .gitignore to match current config
Copy public and private themes during build