shields

This reimplements the idea @bkdotcom came up with in #1519, and took a stab at in #1525. It’s a really powerful way to add all sorts of custom badges, particularly considering [tools like RunKit endpoints and Jupyter Kernel Gateway](https://github.com/badges/shields/issues/2259#issuecomment-444186589), not to mention all the other ways cloud functions can be deployed these days.