RFE: Make --mpv-OPT options apply to running instance

[JJRcop]

Request For Enhancement (Feature request)

Overview Description: Passing --mpv-OPT=VALUE to celluloid while an instance is open does nothing currently. It would be nice for it to inject the option into the running instance temporarily (not updating Extra MPV Options preferences).

Steps to Reproduce:

1. Open celluloid
2. Run celluloid --mpv-play-dir=backward in terminal

Actual Results: celluloid exits with no changes to running instance

Expected Results: The running instance receives the mpv options and applies them, then the sending celluloid command exits like normal.

Version: 0.20

Additional info: You can edit Extra MPV Options in preferences to work around this, but it is saved to gconf settings and persists. This enhancement will allow for ephemeral changes that are lost when the program is closed.