Allow for optional xhr headers object when retrieving data from url

src/config.js

@@ -55,6 +55,7 @@ c3_chart_internal_fn.getDefaultConfig = function () {
         data_onselected: function () {},
         data_onunselected: function () {},
         data_url: undefined,
+        data_headers: undefined,
         data_json: undefined,
         data_rows: undefined,
         data_columns: undefined,

src/core.js

@@ -72,7 +72,7 @@ c3_chart_internal_fn.init = function () {
     $$.initParams();
 
     if (config.data_url) {
-        $$.convertUrlToData(config.data_url, config.data_mimeType, config.data_keys, $$.initWithData);
+        $$.convertUrlToData(config.data_url, config.data_mimeType, config.data_headers, config.data_keys, $$.initWithData);
     }
     else if (config.data_json) {
         $$.initWithData($$.convertJsonToData(config.data_json, config.data_keys));

src/data.convert.js

@@ -1,6 +1,12 @@
-c3_chart_internal_fn.convertUrlToData = function (url, mimeType, keys, done) {
+c3_chart_internal_fn.convertUrlToData = function (url, mimeType, headers, keys, done) {
     var $$ = this, type = mimeType ? mimeType : 'csv';
-    $$.d3.xhr(url, function (error, data) {
+    var req = $$.d3.xhr(url);
+    if (headers) {
+        Object.keys(headers).forEach(function (header) {
+            req.header(header, headers[header]);
+        });
+    }
+    req.get(function (error, data) {
         var d;
         if (!data) {
             throw new Error(error.responseURL + ' ' + error.status + ' (' + error.statusText + ')');

src/data.load.js

@@ -39,7 +39,7 @@ c3_chart_internal_fn.loadFromArgs = function (args) {
         $$.load($$.convertDataToTargets(args.data), args);
     }
     else if (args.url) {
-        $$.convertUrlToData(args.url, args.mimeType, args.keys, function (data) {
+        $$.convertUrlToData(args.url, args.mimeType, args.headers, args.keys, function (data) {
             $$.load($$.convertDataToTargets(data), args);
         });
     }