Support inline html output

[JackDunnNZ]

As suggested in #728 (comment)

I'm unsure about the best order of the various outputs, but if text/html is being used for interactive graphics it should probably come ahead of static images in case both are supported.

[mortenpi]

Thanks! I think this is a good thing to have, although I am wondering if this should be an opt-in? Is there a case where you might want to show text/plain instead of text/html, even if it has a corresponding show method defined?

• As for the ordering -- text/html should be the very first one, I guess? E.g. for interactive plots. Also, I believe Jupyter/IJulia has this hierarchy somewhere defined as well, so it might be a good idea to be consistent with them.
• There should be a test for this in test/examples/. You can look at Capture PNG output in addition to SVG #738 / Add inline WebP/GIF/JPEG images #759 to see how it was done for the other formats. Some simple SVG+HTML+JS combo might be best as a proof of concept.

[JackDunnNZ]

Yeah, I am not sure about the overall organization and fallbacks etc, your opinion would be useful. I think IJulia asks you to use display to show a different mime, so maybe that is a good approach to take here, i.e. finishing the block with

display("text/plain", x)  # hide

I have updated to match the ordering in IJulia as listed here

Also added a simple example SVG with some JS interactivity

[fredrikekre]

I think this is fine, most objects does not define a show for html anyway, and those who do probably want them to be used.

[mortenpi]

Agreed. If there are some corner cases, we can deal with them then.