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. 
Although, given ` /usr/local/lib/ipe ` is just a symlink back to ` /usr/local/Cellar/ipe/7.1.5/lib/ipe/ ` how necessary is this? Can ` ipelets ` not be installed into the ` Cellar ` prefix alongside the regular ` ipelets ` without having to modify ` ipe ` itself? And ` ipe ` would presumably accept them there? Unless I'm misunderstanding your intent here. This comment [relates to this one further below](https://github.com/Homebrew/homebrew/pull/35222#discussion_r22241488).