Fix the markdown document to the new advice for graphing

[jlperla]

Currently https://github.com/econtoolkit/julia/blob/master/plotting.md gives the advice which we need to update.

The current advice for basic users: Use Plots.jl with the gr() backend. It is the easiest to setup, even if not always the prettiest.

The prettiest for print, if setup correctly, is PyPlot directly or the pyplot backend. But it is tricky to get installed on a desktop.

The plotly back-end and PlotlyJS libraries are pretty, but they also have trouble with jupyter at this point.

[jlperla]

Will need to consider the QueryVerse additiosn as well.