Enhance savefig()

[alyst]

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This PR does 2 things:

• cleanup savefig(): combine docstring from all 3 versions, streamline keyword argument passing using kwargs...
• add plotlyjs and plotly_version keyword arguments to savefig() to control which plotly.js Kaleido would be using to generate the figures. This is to allow using more recent versions of plotly.js than the one provided by the PlotlyJS.jl (the current one, 2.3.0, is more than a year old).

I have also regenerated the plotly.js artifact using version 2.35.2. I can provide the file, or the maintainers can run generate_artifacts.jl themselves.

See also comment for PlotlyBase.jl#106

[alyst]

Looks like windows tests fail due to kaleido 0.2 issue not related to PR. Documentation fails because some examples fail to access certain URLs.

[jd-foster]

Thanks for the PR, I'll try to take a look soon.

[BeastyBlacksmith]

Would this adress https://github.com/JuliaPlots/PlotlyJS.jl/issues/503 ?

[codecov-commenter]

Codecov Report

:x: Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 51.81%. Comparing base (40caf9c) to head (5b3fbf8). :warning: Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/kaleido.jl	83.33%	3 Missing :warning:
src/display.jl	0.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #494       +/-   ##
===========================================
+ Coverage   28.67%   51.81%   +23.13%     
===========================================
  Files           8        8               
  Lines         265      276       +11     
===========================================
+ Hits           76      143       +67     
+ Misses        189      133       -56     

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[jd-foster]

We will fix the documentation failure (not related to this PR) in a separate PR