Adding subtitle support via workaround

[akazukin5151]

As noted in #331 and #964, there are no subtitles when opening videos with mpv through mpsyt. Since mpv [link] does show subtitles, the problem is likely in how mpsyt calls mpv, but I'm not sure exactly where. A temporary workaround is to add a new command, similar to the 'copy link to clipboard', except that the link goes to opening mpv directly. In the video list, typing m <number> will do mpv [link] and subtitles will now work.

This isn't the ideal solution (it imports os, check for whether mpv is installed is not implemented, no mplayer support), but there's hasn't been any progress on adding subtitle support, so this workaround might be useful for people at the meantime

[tommysolsen]

If just passing the video url is enough to have it display subtitles, i guess either the way the video url is resolved and passed to the MPV player class or some flag we're setting that is hindering them from being shown correctly.

This would definitely be something that is best off being a part of the core video watching flow, as the extra functionality seemingly often gets overlooked, though if no one steps up and is willing to submit a PR fixing the issue in the main mpv player, then I'dd be happy to merge this.

Gonna leave it for a short while and see