Add loopback:swagger generator

/to @bajtos @ritch @altsang 

The PR adds a new sub-generator to loopback to generate models and skeleton remote methods from swagger specs (v1.2 and 2.0).

- Asks for a url or path to the swagger spec json document
- Loads/parses the swagger spec. 
- Builds a list of models and js files that contain the skeleton remote methods.
-  Checks the existence of models to mark if a model needs to be overwritten and prompts the user to select models using the checkbox
- Prompts the users to select a data source for the models from the list of configured data sources
- Calls workspace apis to generate model defs, configs, and js files

Issues found for workspace:
- https://github.com/strongloop/loopback-workspace/pull/131  
- There is no way to reference a model from common/models from a js file under server/models
I am wondering what is the correct behaviour when you are updating en existing model. ATM, you are overwriting the properties, dropping any extra properties that may have been added by the user. Should you preserve these custom properties instead?