Rewrite CLI

[blaix]

It' still on Gren 0.4.5, and I'd like to move away from shikensu-gren now that Gren has better support for working with the filesystem.

Instead of a separate gren project, rewrite in main project to support the following:

• [ ] gren run PN: show help output
• [ ] gren run PN.Init
• [ ] gren run PN.Build
• [ ] gren run PN.Build --optimize (or -- --optimize?)

eventually:

• [ ] gren run PN.Generate: show help output
• [ ] gren run PN.Generate.Route
• [ ] gren run PN.Generate.Component