v3.1.0

• Add d3.symbolsStroke. Thanks, @hemanrobinson! #176 #189
• Add d3.symbolsFill, replacing (deprecating) d3.symbols.
• Add d3.symbolAsterisk.
• Add d3.symbolDiamond2.
• Add d3.symbolPlus.
• Add d3.symbolSquare2.
• Add d3.symbolTriangle2.
• Add d3.symbolX.
• Add d3.link.