Add support for HTML saving via MIME types

[davidanthoff]

This will enable saving of vega and vega-lite files to HTML.

I've added a custom MIME type called application/vnd.julia.fileio.htmlfile for situations where a type does not want to provide a show method for text/html, but still wants to support saving as HTML files (this is a case I need to support in VegaLite.jl).

[timholy]

Willing to merge this in principle (this is an area I don't know a lot about), but the tests are currently failing. Does it just need to be retriggered or is there something missing here?

[davidanthoff]

Ok, tests are fixed now!

[codecov]

Codecov Report

Merging #250 into master will decrease coverage by 0.13%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   76.22%   76.08%   -0.14%     
==========================================
  Files           8        8              
  Lines         408      414       +6     
==========================================
+ Hits          311      315       +4     
- Misses         97       99       +2

Impacted Files	Coverage Δ
src/registry.jl	92.64% <ø> (ø)	⬆️
src/mimesave.jl	72.72% <66.66%> (-2.28%)	⬇️

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[davidanthoff]

Thanks! Could you also tag a release?

[timholy]

JuliaRegistries/General#9078