For colour in MFM attributes, we expected a `#`, but that's apparently wrong. The BE
translates the `color` attribute in `$[fg.color=000 text]` into `data-mfm-color=000`.
But for the SCSS to work, we need to put it in the style attribute as `--mfm-color: #000`.
Generally we just add the attribute value as-is in the `style` attribute, but now we
have a special exception for color so we add a `#` before the value.
We take the value from a data-* attribute and then add this to the style attribute.
This will probably be OK in most cases, but just to be sure, we check for "weird" characters first.
For now we only allow letters, numbers, dot, hash, and plus and minus sign, because those are the ones I currently know of who are used in MFM.
The data-* attribute remains because it was already considered proper HTML as-is.
