Allow Control-C to gracefully exit a Facedancer emulation

[antoinevg]

This PR attempts to resolve the tension between:

• Being able to exit an emulation in order for a) the --suggest argument to work and b) just having something nicer than crashing out of the emulation with a SIGINT versus…
• Not interfering with any external scripts running one or more emulations.

I propose that we dedicate an unused key-combination (at the time of writing this is Control-E) that can be polled for in an exit_handler() co-routine.

The final approach taken is to catch the KeyboardInterrupt exception in the default_main() helper function allowing users to gracefully exit from the emulation by pressing Control-C.

For users who may require different behavior a new example has been added: examples/coroutine.py which demonstrates how to write a custom default_main function and the use of coroutines.

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Closes #118