feature: synthesize lowercase anchors for heading

[adonovan]

GitHub's markdown renderer creates lowercase anchors for headings. For example, this heading, ## Diagnostic, can be found using either of these two URLs, which differ in the case of their fragment ID:

https://github.com/golang/tools/blob/master/gopls/doc/settings.md#diagnostic
https://github.com/golang/tools/blob/master/gopls/doc/settings.md#Diagnostic

Perhaps your markdown renderer (which has been really useful--thanks!) could do the same.

[rsc]

Is the GitHub algorithm for turning heading text into anchor IDs documented somewhere?
I don't mind adding it as an option if I can find the spec.
Not sure about two choices, but one automatic choice would be good.
(GitHub actually supports any casing at all: https://github.com/golang/tools/blob/master/gopls/doc/settings.md#DiAgNoStIc
Clearly JS is doing that.)