Fix converter custom parameter builder (AudioSegment.from_file)

[pedrohml]

Shifts the standard output (-) parameter to be the last one in the converter parameters so custom input parameters can take effect for conversion using ffmpeg.

I have faced issues when using the parameter -map to select audio streams for conversion as it only take effect if before stdout parameter.

Wrong:

ffmpeg -y -i input -f wav - -map 0:1 > test-wrong.wav

Right:

ffmpeg -y -i input -f wav -map 0:1 - > test-right.wav

In the examples above, only test-right.wav is successfully converted given the selected stream index (i.e. 1).

This follows the established usage instructions where the output destination should be the last parameter to specify (i.e. output_url):

ffmpeg [global_options] {[input_file_options] -i input_url} ... {[output_file_options] output_url} ...

[jochie]

Ran into the same problem (and was putting together an identical PR) when doing exactly what API.markdown suggests we should be able to do, parameters=["-ac", "2"].

Would be lovely to get this merged. :)

[sarangkkl]

LGTM