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've joined @bnoordhuis in thinking people's personal editor ephemera should be in their .gitignore, otherwise all of our git repos .gitignore files become archives of the temp file patterns of every editor ever used by a committer.