Add a way to generate a model in an existent project, instead of the complete project

[TiagoSilvaPereira]

Add a way to generate a model in an existent project, instead of the complete project. Needs to:

• Create a clear interface to enter the model information and your attributes. (* Keep yml?)
• Keep the model indexes on the pwc-gen.json file
• Verify if a model with the same name was generated. If true, use the same index and rewrite the files.

• Verify a way to add multiple models with relationships. I think something like:

pwc models -f ...

or

pwc section -f ...