app: implement the main generator

Implement `yo loopback` as a thin wrapper around loopback-workspace's
`Project.createFromTemplate`. Allow the user to choose a custom project
name and select the project template he wants to use.
model: implement the Model generator

Implement `yo loopback:model` that calls Project methods to update
the project files and define a new Model.
fixup! app option --skip-install
1.0.0-beta1
Merge branch 'release/1.0.0-beta1' into production
Merge tag 'v1.0.0-beta1'

1.0.0-beta1