Add console command

This pull request is **not** meant to be merged, I just want to start a discussion about what I'd like to do. I wrote a naive draft of a `spin console` that would, unsurprisingly, fire up a new rails console in the preloaded app. 

This version is working but is far from being ready. It lacks:

- tests
- documentation
- clean-up/refactoring due to code duplication

And maybe it's missing something else I can't see now. So if you like the idea of having `spin console` I'll proceed with it and take care of the lacking parts. It would be a pleasure to add such a nice command to a project I like/use so much. Furthermore, I'd like to have your opinion how you think we should test the feature and if you think there is a better/different approach I might have to consider to achieve the same goal.

One thing that is worth noticing is that I had to change pre-loading from `config/application.rb` to `config/environment.rb` to actually have the app loaded in the console. Not sure if this can be handled in a better way.

If we are going to finally merge it, this pull request would close the issue #10.