Add option to execute post-processing command after acquiring an episode

[janw]

This could be implemented as an --exec argument or as a config file entry, and might be useful to e.g. convert downloaded episodes to another, preferred, audio format using FFmpeg:

--exec ffmpeg -i {filename} -c:a mp3 -b:a 64k -ac 1 mono.flac {basename}_mono.mp3

Parsing of the option should follow popular tools with similar functionality, such as find.