Add missing reserved words so compiler knows to use array syntax:

* await
* null
* true
* false

IE 8 was failing to compile Handlebars-generated source code
because it had helpers.null.

I came up with this list by diffing
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Keywords
against the ones Handlebars already had.

I added two corresponding tests for nameLookup.

lib/handlebars/compiler/javascript-compiler.js

@@ -1025,7 +1025,8 @@ var reservedWords = (
   " class float package throws" +
   " const goto private transient" +
   " debugger implements protected volatile" +
-  " double import public let yield"
+  " double import public let yield await" +
+  " null true false"
 ).split(" ");
 
 var compilerWords = JavaScriptCompiler.RESERVED_WORDS = {};