.liftoffrc

Here's a first _draft_ of the `.liftoffrc` feature. See #47.

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

- [x] .liftoffrc validation
- [x] Nicer output for the user
- [x] Update README
- [ ] Write in "proper" Ruby 

Feedbacks? (_Please forgive my Ruby_)

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Some new tasks after the first code review round.

- [ ] Use parenthesis for methods names.
- [ ] Use instance methods rather than class.
- [ ] Use YAML instead of JSON.
- [ ] Refactor _options getter_ method to override default option with `~` options, the override those with `pwd` options.
- [ ] Create file with default options
- [ ] `.rspec` + Rakefile update
- [ ] Restore `liftoff -a` behaviour, and plug options inspection on just `liftoff`.