More plot packages support in RMarkdown document

[Non-Contradiction]

Description

This PR wants to support more Julia plot packages in RMarkdown document like PyPlots.

Related Issue

#121
#123

Example

If the PR is finished, then things like

using PyPlot
# use x = linspace(0,2*pi,1000) in Julia 0.6
x = range(0; stop=2*pi, length=1000); y = sin.(3 * x + 4 * cos.(2 * x));
plot(x, y, color="red", linewidth=2.0, linestyle="--")
title("A sinusoidally modulated sinusoid")

should give a plot in a julia chunk in an RMarkdown document.