add arrow function snippets to js-mode (ES2015 syntax) (#226)

* add arrow function snippets to js-mode (ES2015 syntax)

For details, see below link:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

* Make 'arrow-function' snippet more simple

snippets/js-mode/arrow-function

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: arrow function
+# key: af
+# group: es6
+# --
+(${1:arg}) => $0