FFmpegWebVideo external subtitle support + more

EDIT: oops, wrong branch, resubmitted. how do you delete a pull request?

~~this is the minimal patch to get web external subtitles working, plus it fixes the quote-wrapping error (and enables rtmp support if and when you add it to ffmpeg).

There's also an alternative version to unify ffmpeg command creation in FFmpegVideo.java which is working for me but is really just a first pass for valib to consider here https://github.com/skeptical/UniversalMediaServer/commit/296b2b1fbec619a4ff567a1ed56664ca63c106f0~~