Feature Request: Add a new property (time_scale) for the Scene class.

[BreakingLead]

Description of proposed feature

New property named time_scale for Scene class.

How can the new feature be used?

If self.time_scale = 0.5, self.play(run_time=10) will run like self.play(run_time=5) in order to render faster in debugging stage.

Additional comments

Maybe I should initiate a pull request?

[ubaldot]

Why don't just do the following?

time_scale = 0.5
self.play(run_time=10*time_scale)