add Promise snippet to js-mode (since ES2015) (AndreaCrotti#228)

For details, see below link:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/promise

Author: ybiquitous

snippets/js-mode/promise

@@ -0,0 +1,8 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: Promise
+# key: prom
+# group: es6
+# --
+new Promise((resolve, reject) => {
+  $0
+});