Add HTML support for showing images

[johnnychen94]

Can't push to #49 so I open a new PR here. Changes on top of #49:

• set default format PNG, because PNG is lossless and supports transparency.
• reuse the show_element codes
• "fixes" the test on 1.0: this is because ImageIO is only available on Julia >= 1.3 and thus reference results may change.

closes #48
closes #49

cc: @lorenzoh

[codecov]

Codecov Report

Merging #50 (6b8583d) into master (324e8a4) will increase coverage by 0.09%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   87.89%   87.98%   +0.09%     
==========================================
  Files           6        6              
  Lines         223      233      +10     
==========================================
+ Hits          196      205       +9     
- Misses         27       28       +1     

Impacted Files	Coverage Δ
src/ImageShow.jl	0.00% <ø> (ø)
src/showmime.jl	95.55% <93.75%> (-0.70%)	⬇️

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[lorenzoh]

Looks good to me 👍

[johnnychen94]

This might break some people's workflow, I'm not sure. But let's check it out in the wild JuliaRegistries/General#59456

[fonsp]

This does not work well in Pluto notebooks, where suddenly our specialized PNG display (with native performance and nice scaling and reduced layout shifts) is replaced by a blurry small inline image 😭

We use Images.jl extensively in https://computationalthinking.mit.edu/ , where the automatic scaling causes confusingly small and blurry images. I now have to add an override to disable this PR:

Base.showable(::MIME"text/html", ::AbstractMatrix{<:Colorant}) = false

@lorenzoh can I work with you to add the PNG->HTML to Pollen.jl directly, instead of to this package? Or maybe we can hide this change behind a flag?

[fonsp]

Another example from the 3b1b lecture:

Before

Schermopname.2022-05-11.om.20.36.35.mov

After this PR

The image is too small, and you often get vertical layout shifts when the <img> element is replaced by the new HTML output:

Schermopname.2022-05-11.om.20.35.14.mov

Schermopname.2022-05-11.om.20.34.27.mov

[johnnychen94]

Oh, this is so unfortunate! I knew there will be some conflict if the same HTML output feature is defined in downstream packages but didn't expect it to be undesired as this (especially the vertical layout shifts).