Define showable for PNG MIME type

[dahtah]

trafficstars

Currently only showable("image/svg+xml",Figure) is defined. Adding showable("image/png",Figure) = true should enable third-party display systems like ElectronDisplay.jl to work with PyPlot.jl out of the box.

[jebej]

showable is defined for pngs, can you try running methods(Base.showable)?

I get this in the 7th entry:

[7] showable(::MIME{Symbol("image/png")}, f::Figure) in PyPlot at C:\Users\Jeremy\.julia\packages\PyPlot\XHEG0\src\PyPlot.jl:86