Mock once feature

[zifeo]

Hey, it would be very convient to have a utility function to mock the route and remove the route straight after it gets 1 call. Maybe even a mock_n one.

[clo4]

I like it! Can immediately see why that's useful.

Technically this could be implemented entirely as a wrapper function, but including in the library itself would be way better.

I'm planning to update the router dep (see #2) but hit some unexpected trouble with that. Once that's done I'll introduce this feature.

Also planning to take a look at the API surface of this module... I don't love it right now. Would appreciate your input on how it could be improved!

[zifeo]

@separaterecords Thanks for getting back. I believe it would be really useful to have also something to check whether the routes were called or not thus enforcing a stronger test process.