* Add usemap attribute to image macro
Allow for a usemap attribute on the $image macro call which is passed through to the resulting img tag. This makes the use of HTML image maps [1] possible.
[1]: <https://www.w3schools.com/html/html_images_imagemap.asp>
* Document new usemap attribute in ImageWidget
* Update version docs
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Co-authored-by: Jeremy Ruston <jeremy@jermolene.com>
A bunch of little changes that together enable external image support.
Try:
```
tiddlywiki editions/tw5.com --verbose --build externalimages
```
Then open `externalimages.html`, look for the images in the more/types
tab of the sidebar, open them and verify that they are set with an
external SRC attribute, not a data URI.
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