demux_mkv: always add subtitle tracks

Even if the codec is unknown.
core: display subtitle codec in track listing

Also switch the subrip and subviewer names, which obviously have been
confused.
subassconvert: add more web colors

Now includes the complete list from [1] at this time.

Switch from BGR to RGB to match with that list.

[1] http://www.w3.org/TR/css3-color/#svg-color