Request: Add options to trim seconds

[LaurentKinomap]

Seems like ffmpeg does not create the correct bin file when using the parameters -ss (to start extraction at a specific second) and -t (the length of video to take from the start point).
Thus it would be interesting to be able to tell gopro2gpx to trim the GPX file with the same 2 parameters instead.
./gopro2gpx -i tmp.bin -ss 20 -t 10 -o tmp.gpx
would extract the GPS starting at second 00:20 for a length of 10 seconds