Support attributes with { }

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I'm trying to add a class (and other attributes) to images, but it isn't rendered as I thought it would.

The class I put with `{ .onethird }` just after the image is rendered as text following the image:

This is very useful to add classes, ids and other attributes, and often used when the Markdown renderer is Markdown-it: https://www.npmjs.com/package/markdown-it-attrs

The issue names no source file or test; start by locating MacDown’s Markdown-to-HTML rendering entry point and checking how `{ .onethird }` is parsed after an image. Done means the class and other attributes are rendered as HTML attributes rather than literal text, with a regression test.

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