Add Podcast Index as a podcast directory source

[coldacid]

Please add support for Podcast Index as one of the sources for the podcast directory. I was looking at possibly adding it via plugin, but it seems that the plugin API does not support adding additional sources.

[Steven]

thats added for v3.4
https://getmusicbee.com/patches/MusicBee34_Patched.zip
unzip and replace the existing musicbee application files