First working draft of .liftoffrc

Here's a first _draft_ of the `.liftoffrc` feature.

The file is a simple JSON, the `turn_on_all_options` method looks for it first on `pwd`, then on `~`, and if it doesn't find it falls back to the default set of settings.

I think JSON is a natural and smart option for this, but other format could be valid as well, YAML maybe?

Some things left to do

[ ] .liftoffrc validation
[ ] Nicer output for the user
[ ] Update README
[ ] Write in "proper" Ruby 

Feedbacks? (_Please forgive my Ruby_)