Default to SVG output in VSCode & Atom

Currently, svg output is disabled by default because it is really slow in IJulia, something I have tested for myself. However, displaying svgs in e.g. VSCode works well, and gives the user sharper plots as a result.

Could we switch the default in this package to display svg, and either have IJulia itself disable the svg output, or PyPlot detect that it is running within an IJulia kernel?

Start by tracing PyPlot's default SVG setting and how output is handled in an IJulia kernel, then compare the behavior expected in VSCode and Atom. Done means SVG is the default for supported editor use while IJulia avoids the reported performance problem, with the behavior covered by relevant tests if they exist.

Written by the indexing model from the issue text.