Infinality module

There are many Infinality settings which are intended to be
user-configurable, but the Infinality settings resided in the read-only
Nix store. This module allows those options to be configured and
documents them. Some enviroment variables must also be set according to
the desired style to activate the Infinality FreeType patches. The many
style presets that ship with Infinality are exposed by this module.

@vcunat We talked about this last month. This is one of the modules I've been using for some time to properly control Infinality and Fontconfig. This module will fix the problem where Infinality was enabled in Fontconfig but disabled in FreeType. It defaults to "on" in both places, but disabling it in the module will now *actually* disable it in Fontconfig.