Add command for setting secondary subtitles

[leiserfg]

It could be the same as "uosc/subtitles" but for setting --secondary-sid={the_id}. Which is very useful for language learning.

[GunGunGun]

+1, I would love to have a sub-menu inside subtitle picker menu to pick 2nd sub, it's so so useful for language learning.

[k1gen]

+1