Get film duration in H:M:S format

Easier to add `-sexagesimal` flag to ffprobe [in server.R](https://github.com/frenchja/streamR/blob/4de7d16e42691127ebc1c8d155521aebc53edaaa/server.R#L26-L32). Then convert to time using [`as.POSIXct()`](https://stat.ethz.ch/R-manual/R-devel/library/base/html/as.POSIXlt.html) or [`hms()`](http://www.inside-r.org/packages/cran/lubridate/docs/hms).

See https://trac.ffmpeg.org/wiki/FFprobeTips
