Add '-map 0' option to convert multiple audio track in a single file

[petrkotek]

:+1:

Thanks for that, Matt

for the record: I checked the documentation of ffmpeg (https://www.ffmpeg.org/ffmpeg.html#Advanced-options) and -map 0 will map all streams from the input file to output, so it should solve the issue https://github.com/bc-petrkotek/chromecastize/issues/2