Introduce Page.injectFile method

With this patch, page has two methods to include javascript:
- Page.addScriptTag(url) which is similar to phantom's includeJs.
- Page.injectFile(filePath) which is similar to phantom's injectJs.

lib/Page.js

@@ -66,7 +66,7 @@ class Page extends EventEmitter {
      * @param {string} url
      * @return {!Promise}
      */
-    async addScript(url) {
+    async addScriptTag(url) {
         return this.evaluateAsync(addScriptTag, url);
 
         /**
@@ -81,6 +81,15 @@ class Page extends EventEmitter {
         }
     }
 
+    /**
+     * @param {string} filePath
+     * @return {!Promise}
+     */
+    async injectFile(filePath) {
+        var scriptContent = fs.readFileSync(filePath, 'utf8');
+        await this.evaluate(scriptContent);
+    }
+
     /**
      * @param {string} name
      * @param {function(?)} callback