Add property "authorization" to the MetaConfig interface

This property isn't properly documented (https://github.com/systemjs/systemjs/blob/master/docs/config-api.md#meta),
but the implementation can be found here:
* 96f9ec1edf/src/fetch.js (L26)
* 96f9ec1edf/src/fetch.js (L83)

types/systemjs/index.d.ts

@@ -111,6 +111,12 @@ declare namespace SystemJSLoader {
          * Use with the SystemJS Builder. (https://github.com/systemjs/builder#ignore-resources)
          */
         build?: boolean;
+
+        /**
+         * A truthy value enables sending credentials to the server on every request. Additionally, a string value adds
+         * an "Authorization" header with that value to all requests.
+         */
+        authorization?: string | boolean;
     }
 
     interface PackageConfig {

types/systemjs/systemjs-tests.ts

@@ -22,6 +22,22 @@ SystemJS.config({
     }
 });
 
+SystemJS.config({
+    meta: {
+        '*': {
+            authorization: true
+        }
+    }
+});
+
+SystemJS.config({
+    meta: {
+        '*': {
+            authorization: 'Basic YWxhZGRpbjpvcGVuc2VzYW1l'
+        }
+    }
+});
+
 SystemJS.config({
   map: {
     'local/package': {