add target option and coverage to runtests

runtests [TARGET]

TARGET is assumed to be importable as "cherrymusicserver.TARGET" and
testable as "cherrymusicserver.test.test_TARGET". If not given, the
whole codebase will be tested, excluding a potential local copy of
cherrypy.

Coverage will be limited to the TARGET, if one is given. In that case, a
coverage of 100% is required to pass. If there is no TARGET, coverage
will be limited to the cherrymusicserver, cmbootstrap and audiotranscode
packages and require a minimum of 50% to pass. If the coverage plugin
can't be imported, no minimum coverage is required.

(cherry picked from commit cd152c2559e0081e6de5d7f30b67ac7902fc9ce3)