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.
I'm waiting for @bajtos to convert it to a sub generator of loopback, such as `slc loopback:angular`.