feat(loader): runtime exports an object in extract mode

In extract mode generated runtime exports object with id, viewBox, url fields and with toString
method. That allows to use svgs from external sprite when rendering symbols by js.

BREAKING CHANGE:
Generated runtime in extract mode is changed from string to object

ISSUES CLOSED: #123

lib/runtime-generator.js

@@ -22,11 +22,16 @@ function runtimeGenerator(params) {
 
   if (extract) {
     const spritePlaceholder = generateSpritePlaceholder(symbol.request.file);
-
-    runtime = generateExport(
-      `__webpack_public_path__ + ${stringify(spritePlaceholder)}`,
-      esModule
-    );
+    const data = {
+      id: symbol.id,
+      viewBox: stringify(symbol.viewBox),
+      url: `__webpack_public_path__ + ${stringify(spritePlaceholder)}`,
+      toString: function toString() {
+        return this.url;
+      }
+    };
+
+    runtime = generateExport(data, esModule);
   } else {
     const spriteModuleImport = stringifyRequest({ context }, spriteModule);
     const symbolModuleImport = stringifyRequest({ context }, symbolModule);