Implement property generator + move value providers to loopback-workspace

Implement `loopback:property` generator for adding new model properties. Modify `loopback:model` generator to repeatedly ask for model properties.

Refactor `loopback:datasource` and `loopback:acl` to use value providers from loopback-workspace.

Requires strongloop/loopback-workspace#48.

/to @ritch please review.
/cc @sam-github 

This pull request finishes the initial conversion of `slc lb` to `yo loopback`.