Live reload when yaml changes

[lordgreg]

If taskcrafter is already running and has jobs loaded from the file, we don't have other option than stopping the execution, updating yaml, then starting binary again.

Add the ability to live-reload the config even if binary is running.

Side-effects or consider brainstorming about:

• what happens with jobs that are currently running? (shutdown all jobs)
• what happens if yaml invalid while updating? (probably output stderr but continue running binary)
• workflow (verify > stop all > re-execute)