Custom singleton implementation for MetaTagRegistry.

By default MetaTagRegistry.instance will return a registry that's
loaded with builtin tags.  This instance can be overridden for
testing and for loading custom tags.

This greatly simplifies tests and makes them faster as we no more
need to ensure that MetaTagRegistry has been loaded with builtins
before running the tests.  The registry is only instanciated once
for the general case and second time for running special tests
against itself.  Previously initialization happened multiple times.