Add "--force-mono" or "--disable-phase-inv" to opusdec

[TurboCapybara]

As specified on the opus update on section 10 the decoder can disable phase inversion so that downmixing to mono sounds good.

I suggest a "--force-mono" option with the phase inversion already disabled, so that decoding to mono doesn't sound bad.

As an example, ffmpeg does have an option to disable phase inversion (-apply_phase_inv false) but it doesn't downmix it to mono until "-ac 1" is used before the output.

I would make a pull request, but I don't know how to code yet 😢