Separate theme coloring for link text and url

Nobody has claimed this yet.

It seems that there is no way to separately style the text and url of markdown links with MacDown. I'd love to give the link itself a separate color from the text, in other words have `[blue](light-blue)` kind of styling. But it seems we only have `LINK` available in the theme.

Could you add a `LINK_URL` option to color the url separately? If the `LINK_URL` color is missing, just use the `LINK` color, but if it is present in the theme, then use it for the url portion of the link.

This would make it much easier to read documents with lots of links. What most people really need to do is read either the primary text (proofreading for meaning) or the urls (proofreading for function), but those are two very different tasks. It would be very helpful to use color to distinguish those portions of the document.

No files or tests are named in the issue. Start by locating the theme-rendering code that handles LINK colors, then trace how link text and URLs are styled; done means a LINK_URL theme option colors the URL separately and falls back to LINK when it is absent.

Written by the indexing model from the issue text.