player: make --shuffle/--merge-files affect runtime loaded playlists

Until now, these options took effect only at program start. This could
be confusing when e.g. doing "mpv list.m3u --shuffle". Make them always
take effect when a playlist is loaded either via a playlist file, or
with the "loadlist" command.
player: filter tags, add --display-tags option

This attempts to increase user-friendliness by excluding useless tags.
It should be especially helpful with mp4 files, because the FFmpeg mp4
demuxer adds tons of completely useless information to the metadata.

Fixes #1403.