#14 Allow converting from multiple source formats

[Xiloynaha]

The attached patch fixes the following bug. It may not be the best solution, as I'm not familiar with the source code, but you get the idea.

% ls
myflac.flac  myogg.ogg

% pacpl -t mp3 myflac.flac myogg.ogg

Perl Audio Converter - 5.0.1

Converting: myflac.flac -> mp3
Converting: myogg.ogg -> mp3
Can't use an undefined value as a subroutine reference at pacpl line 2507.

Total files converted: 1, failed: 1

% ls
myflac.flac  myogg.mp3  myogg.ogg