That would involve expanding this particular script's capabilities far beyond what is needed right now. Down the line I do intend to do exactly as you say (one problem of course, is that if I use external template files then I have to version control the template files if they start getting updated). However, I needed to get something out the door today, and digging through/researching templating modules while simultaneously trying to get my formats right, my parsing right, and my e-mail configuration right was going to require a lot of time that I don't have right now.
One day, if all goes well, I fully intend to evolve this into a completely modular, badass notification program. But for the time being, that amount of work would not be adding value to my immediate task at hand
So, long story short, I'm prioritizing.