Initial setting of state (null/ready/paused/playing) for each input/output/mixer

[matthew1000]

Addresses four ares:

1. Allows the user to specify what state a newly created input/output/mixer is. Previously, all would start PLAYING. Now, an ‘initial_state’ property can allow it to be PAUSED or READY (or even NULL, if somehow that's wanted).
2. Stops a race condition where an input can enter PLAYING before the bus has initialised, which resulted in some events being missed and thus certain elements not being unblocked.
3. Removes an anomaly where the RTMP output would enter PAUSED rather than PLAYING.
4. Documents the config file.

• tests pass
• linting passes
• test written