Should be able to list all available (not necessarily in use) scaffold providers

Right now the only way to "discover" which types of scaffold are installed is to either:

a) Check your installed plugins
b) type: "scaffold setup --scaffoldType <TAB>" and see which options it comes up with as completion proposals.

It would be grand to have a "scaffold list-providers" command, to list and display basic information about each scaffold.

E.g:

$ scaffold list-providers
      * Aerogear: Basic PoH5 & Rest Mobile Scaffolding
      * Faces: JavaServer Faces scaffolding
      * SpringMVC: What it sounds like...