This brings several changes with it:
- `Mpv::get_property` now returns `Option`s in case `data` is
nonexistent. There could be cases where this is different from
`MpvDataType::Null` and `MpvDataType::MinusOne`.
- `MpvError` now implements `PartialEq`
This moves all communication with Mpv's unix socket into another tokio
task, and uses message passing through clonable mpsc channels to receive
commands to execute, and to send responses.
