Override github project for build badge

Hello 🙂
I've included `gitdown` in `check-node-version` for having nicer READMEs.
Since I'm not the original author, I work under a different repo url.

Now, after merging into the original repo, the build badges are wrong because they point to my branch. That was easy to fix, of course, but I'd like to be able have them point at the base repo at build time.

Is that currently possible from within gitdown? If not, I'd like to request that feature.