Add ability for lambda to change the context for fragment execution

src/test/java/com/samskivert/mustache/MustacheTest.java

@@ -664,6 +664,20 @@ public class MustacheTest
              }));
     }
 
+    @Test public void testLambdaWithContext () {
+        test("a in l1, a in l2", "{{#l1}}{{a}}{{/l1}}, {{#l2}}{{a}}{{/l2}}",
+             context("l1", new Mustache.Lambda() {
+                     public void execute(Template.Fragment frag, Writer out) throws IOException {
+                         frag.execute(context("a", "a in l1"), out);
+                     }
+                 }, "l2", new Mustache.Lambda() {
+                     public void execute(Template.Fragment frag, Writer out) throws IOException {
+                         frag.execute(context("a", "a in l2"), out);
+                     }
+                 }
+             ));
+    }
+
     @Test public void testNonStandardDefaultDelims () {
         test(Mustache.compiler().withDelims("<% %>"), "bar", "<%foo%>", new Object() {
             String foo = "bar";