Add force parameter to create

[svdgoor]

Adds a parameter to the briefcase create command to skip confirming overwriting existing scaffolds. This makes it easier to test this part of building, such as when debugging new changes to this system, or to investigate problems when people (like myself earlier today) have an issue in their scaffold.

The tests do not pass anymore. I looked at them, and I see why, but I would rather leave the decision about how to solve them to you. I hope CI will automatically apply to the PR, otherwise I will paste a log of vox here later.