Add IIFE snippet

I'm using this snippet quite often so I figured it might be useful in the general snippet library for this package.

Author: rmehner

snippets/language-javascript.cson

@@ -44,6 +44,9 @@
   'if':
     'prefix': 'if'
     'body': 'if (${1:true}) {\n\t$2\n}'
+  'Immediately-Invoked Function Expression':
+    'prefix': 'iife'
+    'body': '(function() {\n${1:\t"use strict";\n}\t$2\n}());'
   'log':
     'prefix': 'log'
     'body': 'console.log($1);$0'