Added --softdelete option to the migration command

Hey,

I'm a very big fan of yours. Recently I started using Git and GitHub and today while starting a new project of mine I used your generators which are very cool. I wanted to try the softDeletes() in this project but I didn't see such option in the generator, so I decided to add it.

I'll be glad if you consider this pull request.

I'll try to add the option in the model from the generate:resource.