Use charAt rather than string index

Older versions of IE do not support [] access to string contents so charAt must be used.

Fixes #677

lib/handlebars/compiler/ast.js

@@ -19,7 +19,8 @@ var AST = {
     this.hash = hash;
     this.strip = strip;
 
-    var escapeFlag = open[3] || open[2];
+    // Must use charAt to support IE pre-10
+    var escapeFlag = open.charAt(3) || open.charAt(2);
     this.escaped = escapeFlag !== '{' && escapeFlag !== '&';
 
     var id = this.id = rawParams[0];

src/handlebars.yy

@@ -6,8 +6,8 @@
 
 function stripFlags(open, close) {
   return {
-    left: open[2] === '~',
-    right: close[0] === '~' || close[1] === '~'
+    left: open.charAt(2) === '~',
+    right: close.charAt(0) === '~' || close.charAt(1) === '~'
   };
 }