Add custom link handler referencing two fields.

Add custom link handler referencing two fields. Also, update original custom link handler to allow customization of link text as well as URL.

This implements issue #79 (closed) (except for my note about "would be great if the URL could be only generated when the %1 value is not null which I will do a separate PR for")

merged

mentioned in commit b47bda2a

Thanks. I have also updated CHANGELOG.md.