Expose links in badge-maker

:clipboard: **Description**

Fundamentally we need to add the ability to pass a couple of strings into `makeBadge()`, but we need to decide on the interface for that. One of the useful things we've done in v3 is bringing the terminology used in shields.io URL params, the endpoint badge and the NPM package into line. This is **a good thing** but the current interface for links in URL params is a bit weird. Now is probably a good time to re-think it. There is some discussion of this in:

https://github.com/badges/shields/issues/4849#issuecomment-609108141

Of the v3.1 issues, this one is currently the least clearly-defined.

Start with the makeBadge() interface described in this issue and read the linked discussion in shields/issues/4849#issuecomment-609108141. Compare the existing URL-parameter, endpoint-badge, and NPM-package terminology before proposing an interface. Done means the link-passing API is clearly decided and implemented consistently.

Written by the indexing model from the issue text.