`extra_kwargs` tracker

[BeastyBlacksmith]

This issue tracks the implementation of the extra_kwargs functionality added in #2200

• GR
• PyPlot (python depthshade #2703)
• Plotly (Add support for mathjax (latex) and extra_plot_kwargs in plotly backend #2719)
• PlotlyJS
• PGFPlotsX
• InspectDR
• UnicodePlots

[isentropic]

PyPlot, subplot color bar positioning extra kw added

:subplot=>Dict("3d_colorbar_axis" => [0.9, 0.1, 0.03, 0.8])