Allow using code in itemenums

This avoids closing all itemenums after(!) a code section.
use 'or' instead of '|' which is binary or
adding test for using code within itemenums
Merge branch 'code_itemenums'

This is an upgrade version of:

https://github.com/esc/wiki2beamer/pull/6

that adds a test (and fixes a minor semantic issue)

* code_itemenums:
  adding test for using code within itemenums
  use 'or' instead of '|' which is binary or
  Allow using code in itemenums