Extract genenclosure(). No functional change intended.
Render fancy podcast enclosures.

Simple podcast feeds didn't have content tags and I made sure to
keep it that way. This may be unnecessarily conservative. Changing
the behavior to include empty content tags might be fine, but I
don't want to think about it right now, I just want my tests to
keep passing!

The new fancy-podcast tests are copy-pasted-edited from the
simple-podcast tests. These tests shall be refactored.