Is there a way to get this domain to point to my computer's external IP and have XAMPP serve the page to the visitor? I already have it working when the user uses my IP directly, but I want them to be able to use the domain instead.
If you have an internet connection with static IP address, you can do it.
First you have to create an entry in the Apache's VirtualHost on xampp with the domain name, and port. Here is a tutorial on creating the vhost entry. http://httpd.apache.org/docs/2.0/vhosts/examples.html
You can then add the a record with the value of the static public IP in cloudflare.
PS, if you don't have a static public IP address, you will have to use some dynamic DNS provision like dyndns which won't allow you to use the cloudflare.
You can accomplish this just fine even if you have a dynamic IP. Check this out.