Add `preferred_sub_format`

[barraIhsan]

Sometimes there are 2 versions of subtitle included in the video file, hdmv_pgs (bitmap, for movie) and ass (imo, better)

As you can see with my current config

preferred_audio_lang=jpn/ja/japanese
preferred_sub_lang=eng/en

It will select the first non-Sign subtitle (default excluded) which is the hdmv_pgs one.

I'd like to have preferred_sub_format option so we can do something like this

preferred_sub_format=ass