libav-bitrate: option for selecting only part(s) of file

[Nikoli]

Please add options for selecting part or parts of files using time and size. Helpful if you want to check only small questionable part of large file.

[silicontrip]

The only benefit this has is that it produces a smaller output file.
And reduced processing time if the selection is towards the start of the file.

However the program will still have to process all the frames preceding the section.