Add a first name greeting to the new registration email

As much as I try to make the plugin as flexible as possible with a framework that includes lots of hooks for customization, every once in awhile a question pops up that forces me to think outside the box, so to speak.  Case in point, I recently received a question in the forum about putting a first name greeting into the body of the welcome email that goes to a user upon new registration.  Seemingly intuitive and simple, that particular field is not readily available to the email function.  BUT… have no fear, there is a very simple customization that makes it (and other fields) available to the email filter.