Increase support for color aesthetic

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• adds extra support for the syntax color=[colorant"color"]
• fixes Mixing colorants with Scales #1437
• Gallery plots updated. Now there are only a few plots which require the syntax
  layer(..., Theme(default_color= )) e.g. Geom.line doesn't directly support single length color.

Example

D = [DataFrame(x=rand(20), y=rand(20), v=g) for g in ["Group1", "Group2"]]

plot(Coord.cartesian(fixed=true),
      layer((x,y)->x+y, 0, 1.0, 0, 1.0),
      layer(D[1], x=:x, y=:y, color=[colorant"blue"]),
      layer(D[2], x=:x, y=:y, color=[colorant"green"]))