Changes ./bin/test.sh to pass command line options to pytest, and default to
only running tests in the tests/ directory instead of everywhere excluding
a few directories which is more error-prone.
Also keeps the mock_server used in testing quiet so access log entries don't
appear on stdout.
The feedparser packages has 20 years of history and is very good at parsing
RSS and Atom, so use that instead of ad-hoc regex and XML parsing.
The medium_rss and shaarli_rss parsers weren't touched because they are
probably unnecessary. (The special parse for pinboard is just needing because
of how tags work.)
Doesn't include tests because I haven't figured out how to run them in the
docker development setup.
Fixes#1171
