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.
ATM, the problem we are seeing most is permissions problems, not directly caused by peer deps. A lot of people seem to install node in a way that npm scripts can't find it :-(

I dread writing it, but an `slc doctor` to check your node installation might become a requirement...