CLI: `squib add` to add a new deck

[Andy Meneely]

One tedious operation I do a lot is adding a new deck, and when that happens there's a bunch of things that need to be done that could be easily automated. So maybe:

squib add foo

Would result in:

• New file src/foo.rb with defaults in it. Will need basic templating so we can put Foo into it, and use_layout file: foo.yml
• Add task for rake foo to the Rakefile, if it exists
• Add a layout file layouts/foo.yml