Add markers to LinePlot

[mindok]

Add an option to LinePlot for markers.

type: circle, square, diamond, triangle (default to cycle through these in order for each series)
size: (with SVG it’s all relative, but figure out a sensible default)
filled: true or false, default true
colour will be the colour of the line

Note that the plot content protocol will need to be extended to provide "defs" for a given dataset that can be embedded at the top of the SVG ahead of the main plot content.

Options passed in like this:

#show markers with defaults
markers: true 

#show markers explicitly, setting marker properties for each series
markers: [
  [type: :circle, size: 3, filled: false], 
  [type: :square, filled: false],
] 

[megalithic]

@mindok this would be amazing!