Enhancement: purge unused dependencies

[mmindenhall]

I recently evaluated gb and wgo for a new Go project at my company (our first). We chose wgo due to the wgo restore command (we wanted to .gitignore the vendor/src folder).

One thing I really liked about gb was the gb vendor purge command, which "purges all unreferenced dependencies". As we're feeling our way through learning the ecosystem and evaluating competing libraries, I've already had to manually purge a couple of dependencies from vendor.json. It's easy enough to do manually, but it would be a nice addition to wgo.

Thanks!