Requesting configurability of anchor underline lightness ratio

[scottyeck]

Let me start off by saying this package is really really awesome. Typography requires that the design abide by systems that are so practical and yet so difficult to master - at least from my vantage point as a technologist rather than as a designer - so to have all of that bootstrapped is super helpful. Thanks for all your hard work. 👍

Could we add more areas of configurability? For example, by default, anchor underlines are lightened by 20% as compared to the color of the link text - I'd personally love it if this in particular (and several other options, for that matter) were configurable.

Hope this is something you'd be down for. I'm going to submit a PR 😎