Add FlaskWorker

[tonycpsu]

flask_rq2.job.FlaskJob is nice, but initializing the app each time can be time-consuming if the app has to make a database connection, initialize some other extensions, etc. It'd be great if this were handled by a custom Worker class. I gave it a try and while I haven't tested extensively this seems to work:

class FlaskWorker(Worker):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.script_info = ScriptInfo()
        self.app = self.load_app()

    def perform_job(self, job, queue):
        with self.app.app_context():
            return super().perform_job(job, queue)

    def load_app(self):
        if current_app:
            app = current_app
        else:
            app = self.script_info.load_app()
        return app

If this seems useful to anyone else I'd be happy to submit a PR.