G2 should allow inter-G2 links

Currently, G2 only works when the nodes using the DFN element to link to definitions are on the same server as the glossary being used.

It would be useful to enable automatic links to another, remote, G2 glossary instance: that way definitions could be kept on a glossary site, usable by lightweight "client" sites, that could benefit from the existing maintained glossary by just using the G2 input filter.