Allow extending resolution of link definitions

[dmcclean]

Would you support a pull request to mark the GetLinkDefinition function as virtual?

public LinkDefinition GetLinkDefinition(string id)
{
    LinkDefinition link;
    if (m_LinkDefinitions.TryGetValue(id, out link))
        return link;
    else
        return null;
}

I'd like to be able to allow my (internal) users to use a custom syntax for linking to database resources. I think that if it's done by extending the link definition resolution it may be useful to people more generally.

If so, does the project style prefer introducing a template method here so that all subclasses are required to prefer entries in the m_LinkDefinitions lookup to whatever their own virtual entries may be, or is just marking the whole thing virtual OK?