Add support for slc loopback commands that delegate to yeoman

/to @bajtos 
/to @ritch 
/to @sam-github 

The PR adds the support for `slc loopback` commands, for example:

```
slc loopback (yo loopback)
slc loopback:model (yo loopback:model)
slc loopback myapp (yo loopback myapp)
```
The `generator-loopback` becomes a regular dependency to strong-cli.

Unfortunately, it's difficult to add tests here as yeoman needs to receive user inputs from command prompt.
Should it be removed from the peer dep list in the mean time so that we can start fixing people's installs sooner rather than later? Adding it back in as a direct dep later seems lower risk than leaving it in as a peer dep for the next release.