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.
The below could use a comment about what its doing, because its not at all clear to me, and https://github.com/yeoman/generator doesn't document any API, so its not a quick lookup either. I don't know what the namespaces referred to are, and what the aliases are for.

I gather that calling `require('yeoman-generator')()` causes it to examine argv, and then look for and run the generator? And it will find a generator somehow in the local dependency tree, not the global?