Please let me choose the clone URL for slippy submodule

[André Wendt]

The submodule's path is hardcoded in new.rb:4 and points to a repo that hasn't been updated for over a year.

Is there a reason why you don't use the official repository? And if you don't want to by default, please let me choose with an option.

I'd be willing to send you a pull request if you want.

Best,
André

[Owen Ou]

@awendt Yes, please go ahead and do so. The original intention was to have a stable version of slippy.