Pacto generate! and validate! modes

Fixes #52.

Adds:
- Pacto.generate!
- Pacto.generating?
- Pacto.stop_generating!
- Pacto.validate!
- Pacto.validating?
- Pacto.stop_validating!

No stubbing or allow_net_connect at this point.

If this gets a :+1: then I'll simplify the usage section of the readme.

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