options: add --sub-visibility=<primary-only|secondary-only>

Adds --sub-visibility choices 'primary-only' for only displaying the
primary subtitle track, and 'secondary-only' for only displaying
secondary subtitle track.

Removes --secondary-sub-visibility and displays a message telling the
user to use --sub-visibility=yes/primary-only instead.

These changes make it so that the default 'sub-visibility' bind 'v'
cycles through all the 'sub-visibility' choices, 'no', 'yes',
'primary-only', and 'secondary-only'.