mix-to-mono vs stdin and stdout

[janstary]

Both the manpage and mix-to-mono itself say that the synopsis is
sndfile-mix-to-mono <input file> <output file>

Neither of them mention that the inout can be '-', meaning stdin,
and the output can be '-', meaning stdout. All of the following work:

sndfile-mix-to-mono stereo.wav - > mono.wav
sndfile-mix-to-mono - mono.wav < stereo.wav

Of course, the output file type cannot be determined from '-',
but determining the file type from the name doesn't work anyway,
see #93

Also, they cannot both be '-', because

sndfile-mix-to-mono - - < stereo.wav > mono.wav 
Error : input and output file names are the same.

even though the files are not the same of course: it's stdin and stdout.