This fixes some minor problems with the pull request from jvanasco so
that it may be included into the next release:
- Only one extension registry is used internally and custom extensions
do not have to be stored two times.
- Fix the bug that entry extensions were loaded for feeds.
- Do not fail if there is only a feed extension.
- Extensions for entries do not need a feed extension class.
Signed-off-by: Lars Kiesow <lkiesow@uos.de>
303e74dc7a