v0.5.0

• NOTE: behaviour change. If a Dataset is made up of rows of maps, column_names now returns headers if it is supplied, otherwise returns
  map keys from the first row. Previously, it just returned the map keys from the first row.
• NOTE: PlotContent.get_svg_legend has been replaced with PlotContent.get_legend_scales. This moves the responsibility for legend
  rendering to Plot, which allows multiple scales to be combined in future. This change was needed to allow plot content to
  be resized based on the number of entries in the legend.
• Change smoothed line to round coordinates to 2 decimal places (reducing SVG size for large plots).
• Enable plot_options (show x axis, y axis) in LinePlot and PointPlot. Thanks @shadowRR
• Added meta to Dataset to allow carrying of handy information with the dataset.
• Added :legend_top and :legend_bottom options to :legend_settings - note that if you have too many legend entries it
  may push the plot content beyond the bottom of the plot.
• Added ContinuousLogScale. Thanks @l3nz
• Added a gallery into the documentation. Thanks @l3nz
• Added additional examples into the gallery provided by @travelmassive. Thanks @l3nz & @travelmassive
• Fixes to tests & documentation. Thanks @kianmeng, @axelson
• Fix to ContinuousLinearScale. Thanks @ruimfernanded