Add support for pkgstore.skip_bags config

default value is empty list. If list has values, they are
treated as the names of bags to be skipped when reading
from the pkgstore. This is useful for already installed
instances that should not update a particular bag once
installed but twanager update is called.
Merge branch 'master' of github.com:cdent/tiddlywebplugins.pkgstore

Conflicts:
	test/test_write_store.py
	tiddlywebplugins/pkgstore.py