Add rebeccapurple color. Fixes #2208.

d3.js

@@ -1840,6 +1840,7 @@
     plum: 14524637,
     powderblue: 11591910,
     purple: 8388736,
+    rebeccapurple: 6697881,
     red: 16711680,
     rosybrown: 12357519,
     royalblue: 4286945,

src/color/rgb.js

@@ -263,6 +263,7 @@ var d3_rgb_names = d3.map({
   plum: 0xdda0dd,
   powderblue: 0xb0e0e6,
   purple: 0x800080,
+  rebeccapurple: 0x663399,
   red: 0xff0000,
   rosybrown: 0xbc8f8f,
   royalblue: 0x4169e1,