Refactoring yo

Here are the goals:

- Don't use `yeoman-generator`, as yo doesn't generate anything I believe yeoman-generator is not the correct architecture for this project.
- Separate every commands in it's own self contained page route.
- Improve tests: remove network connection, uses correct mock on the low level methods rather than stubbing out used methods.

I extracted a first page (help) for proof of concept. Any further help is welcome on the `refactor` branch.

Keep the project running as we refactor. This mean move pieces out of yoyo.js and into modules, but link them inside yoyo so we keep the product working and testable. We'll work our way to another system pieces by pieces.

#### List of commands

- [ ] Home
- [x] Help
- [x] Run
- [x] Update
- [ ] Clear config
- [x] Search (this one will be broken into two)
- [x] Install
- [x] Exit