openjpeg20 2.0.0

Followup to #283. Adds optional support to `gdal` for JPEG2000, and allows installation of openjpeg for third-party builds.

See also: https://github.com/Homebrew/homebrew/pull/28526

[OpenJPEG 2.1 is out](http://sourceforge.net/projects/openjpeg.mirror/files/2.1.0/), but it seems that nothing in Homebrew can currently use it.