Combining a single file

[manuelhuber]

Hello,

I'd like to use the -v (Combination) command on a single file. While this may seem like nonsense it's an easy way to change the volume of a file by a given percentage, like this:
sox -v 0.9 in.wav out.wav
Currently this is not possible since validate_input_file_list in the file_info.py raises an error when the list has less than 2 items.

While this helps people avoid mistakes it also prevents some functionality. Do you think it is worth removing?

[justinsalamon]

cf #24