Fix SRT/WebVTT subtitle downloads

[scribblemaniac]

This PR most importantly fixes the SRT download option, which was producing invalid output files with a . decimal separator rather than the correct , separator (fixes #118). Additionally this fixes several other errors in the SRT and WebVTT output when dealing with very large timestamps (see commit message for details). It also takes advantage of the fact that the hours are optional in WebVTT to save a couple of bytes on small timecodes < 1 hour.