add link reference option for "no inlines"

[yshavit]

Right now, we have three options for handling link definitions (including images):

1. Keep as-is
2. Always use inline
3. Always use full reference style: [text][1]

In the last option, we do at least collapsed links ([text][]) and shortcut links ([text]) slightly differently, in that rather than just assigning them a number, we have the full-reference id be the expanded text:

• [text][] -> [text][text]
• [text] -> [text][text]

This is pretty ugly and awkward. I should add an option (maybe have it be default? maybe have it be the only option) to keep collapsed and shortcut links as-is, or maybe canonicalize them to either collapsed or shortcut; or maybe just auto-assign a number to them. But basically, the current approach is pretty ugly, especially if the text is a bit longish.

I should also review the various markdown specs to see how portable collapsed/shortcut links are.

[yshavit]

It looks like:

• collapsed links [text][] were in the original markdown spec, called "implicit link name": https://daringfireball.net/projects/markdown/syntax#link
• they do not seem to be in this (unofficial) guide: https://www.markdownguide.org/basic-syntax/#links
• they are in CommonMark: https://spec.commonmark.org/0.31.2/#collapsed-reference-link
• they are in GFM: https://github.github.com/gfm/#collapsed-reference-link