Add new port: devel/mage

Mage is a make/rake-like build tool using Go. You write plain-old go functions,
and Mage automatically uses them as Makefile-like runnable targets.
Makefiles are hard to read and hard to write. Mostly because makefiles are
essentially fancy bash scripts with significant white space and additional
make-related syntax.

Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D15159