Optimize `Writer.prototype.escapedValue` for numbers (#754)

These changes optimizes the performance of escaping number
values, by not passing them through the ordinary HTML escaping
that is done on string values.

mustache.js

@@ -657,7 +657,7 @@
   Writer.prototype.escapedValue = function escapedValue (token, context) {
     var value = context.lookup(token[1]);
     if (value != null)
-      return mustache.escape(value);
+      return typeof value === 'number' ? String(value) : mustache.escape(value);
   };
 
   Writer.prototype.rawValue = function rawValue (token) {

mustache.mjs

@@ -650,7 +650,7 @@ Writer.prototype.unescapedValue = function unescapedValue (token, context) {
 Writer.prototype.escapedValue = function escapedValue (token, context) {
   var value = context.lookup(token[1]);
   if (value != null)
-    return mustache.escape(value);
+    return typeof value === 'number' ? String(value) : mustache.escape(value);
 };
 
 Writer.prototype.rawValue = function rawValue (token) {