Allow SI prefixes to be set in locale.

This could serve as an escape hatch for somebody who wanted to support custom SI prefixes. This could be used to solve d3#33 (binary prefixes) or it could be used to support something custom like American english numbers (1,000,000,000 => 1B [billion] instead of 1G [giga]).

src/locale.js

@@ -4,7 +4,7 @@ import formatSpecifier from "./formatSpecifier";
 import formatTypes from "./formatTypes";
 import {prefixExponent} from "./formatPrefixAuto";
 
-var prefixes = ["y","z","a","f","p","n","µ","m","","k","M","G","T","P","E","Z","Y"];
+var defaultPrefixes = ["y","z","a","f","p","n","µ","m","","k","M","G","T","P","E","Z","Y"];
 
 function identity(x) {
   return x;
@@ -13,7 +13,8 @@ function identity(x) {
 export default function(locale) {
   var group = locale.grouping && locale.thousands ? formatGroup(locale.grouping, locale.thousands) : identity,
       currency = locale.currency,
-      decimal = locale.decimal;
+      decimal = locale.decimal,
+      prefixes = locale.prefixes || defaultPrefixes;
 
   function newFormat(specifier) {
     specifier = formatSpecifier(specifier);