Add ability to include environment variables from external files

I understand the reasoning behind needing to set environment variables in the machine/environment section rather than being able to export them from another command section, but is there a way that we could include environment variables from another file in the machine/environment section itself? I have multiple projects that use the same circle.yml file, only with different project-specific variables set in the machine/environment section, and it would be a big help to be able to just copy changes to my circle.yml file across projects, and have the environment variables set in a separate file that is then included in the circle.yml file.

TL;DR: I want to be able to make my circle.yml files project-independent, and include project-specific variables in an external file for portability.