Escape footnote name

[digitalmoksha]

This escapes footnote names. So [^😄ref] would yield the name %F0%9F%98%84ref

It also makes footnote names case-insensitive but case-preserving. So if the footnote definition is [^aB]: footnote, you can reference it as [^AB] or [^ab], etc. The final name will be the definition name, aB.

Related to #307

[digitalmoksha]

@kivikakk another step on the footnote journey...

There are two commits, one that fixes the encoding of footnote names, the other fixes the case preserving issue.

[kivikakk]

Thanks — excellent. One thing you could consider is making a little purpose-built enum for the second argument, so that it looks more like strings::normalize_label(&nfd.name, PreserveCase::DontPreserve), strings::normalize_label(&nfd.name, PreserveCase::Preserve) — or something like that, I'm not sure about what the best wording would be.

[digitalmoksha]

One thing you could consider is making a little purpose-built enum for the second argument

Ah, I see what you mean. Let me think on it.