Added a "sails issue" command for running one-off commands

The idea is this is for running one-off management commands. 

My motivation is in database management commands/migrations, but it can be used for anything. I plan on trying to use sequelize to introspect the sails models and create migrations/generate schema (hacky associations); the ability to run commands within the context of the app is the first step to that.

Would be happy to hear some feedback on this one!