Show the name of variable that is null or missing

If Compliler.nullValue contains a substring "{{name}}", then this substring
will be replaced with the name of variable.
For example, if nullValue="?{{name}}?" and variable is resolved to null,
then string "?foo?" will be used.

src/main/java/com/samskivert/mustache/Template.java

@@ -170,7 +170,16 @@ public class Template
         // getValue will raise MustacheException if a variable cannot be resolved and missingIsNull
         // is not configured; so we're safe to assume that any null that makes it up to this point
         // can be converted to nullValue
-        return (value == null) ? _compiler.nullValue : value;
+        return (value == null) ? substituteNullValue(name) : value;
+    }
+
+    private String substituteNullValue(String name) {
+        String SUBST = "\\{\\{name\\}\\}"; // escaping needed for regex
+        String result = _compiler.nullValue;
+        if (result != null) {
+            result = result.replaceAll(SUBST, name);
+        }
+        return result;
     }
 
     protected Object getValueIn (Object data, String name, int line)