GPL/non-GPL feature

As some libraries are GPL'd and are in some cases optional requirements of packages, it would be nice if we had a mechanism to control whether such packages do or do not get installed. Related we would need a way of controlling whether a package gets built with a GPL dependency or not. The simplest way that comes to mind ATM would be creating a `gpl` feature and corresponding package that needs to be installed to enable this. This might be too wide sweeping and it may require per package features. If it does start going in the latter direction, perhaps solving issue ( https://github.com/conda/conda/issues/3299 ) will give us something for this too.