CGAL ipelets

This pull request adds cgal-ipelets formula (ipelets which are included with CGAL, but not installed by default). It also fixes a small, but inconvenient bug in the ipe formula. 
Indeed. I don't know how Jack and co feel about the idea of something's Cellar sharing code, but it's certainly not ideal. Most things share a common path in these situations, like Python with ` lib/site-packages ` and Luarocks with ` lib/Luarocks `, etc. Generally, site-wide packages for anything are supposed to have their own path, which the primary depended-on package knows how to find. But ` ipe ` doesn't seem to have that separate directory to find ipelets in capability, outside of accepting them if they're dumped in a hidden $HOME folder?