v0.20.0

Many updates, min julia 1.0

• change display type to use PlotsDisplay (fixes Juno integration)
• change all internal uses of d to plotattributes (no user change)
• change spy implementation to use scatter not heatmap
• sort x axes when passing a vector of strings as x
• improve performance of marker_z
• update CI to 1.0
• minor depwarn ifixes
• only draw one colorbar with GR
• add colorbar_title to GR and pgfplots
• fix savefig with latexstrings for PyPlot
• fix NamedTuple integration
• don't export P2 and P3
• make it possible to use 2-argument function as argument to marker_z
• make plotattr work again