Allow end users to create a blank plugin project with the 'createplugin' command

I thought it'd be worthwhile to write this up, since I'll be making plugins for spreads.

The downside is that this is against upstream/master, so if spreads.plugin changes as per other open issues (#32, #33, #35, #37) this will be out of date.

This branch makes a project directory fine, but its unknown whether the project directory can make a usable plugin!