Update videosub.js

With the current formatting subtitles can be hard to read on white background. To alleviate the problem, I have used textShadow to outline the characters in black. See <https://blogs.fsfe.org/repentinus/files/2013/02/tc_before.png> [before] and <https://blogs.fsfe.org/repentinus/files/2013/02/tc_after.png> [after] for comparison on light background.