Output duration based on -ss and -t flags

[malashin]

Chage output file duration calculation based on -ss or -t flag if present. It is currently based on input file duration.

[malashin]

https://www.ffmpeg.org/ffmpeg-utils.html#time-duration-syntax

There are two accepted syntaxes for expressing time duration.

[-][HH:]MM:SS[.m...]

HH expresses the number of hours, MM the number of minutes for a maximum of 2 digits, and SS the >number of seconds for a maximum of 2 digits. The m at the end expresses decimal value for SS.

or

[-]S+[.m...]

S expresses the number of seconds, with the optional decimal part m.

In both expressions, the optional ‘-’ indicates negative duration.