Implement for-else

This PR implements for-else statements like in Jinja. They make it easy
to print an alternative message if the loop iterator was empty. E.g.

```rs
{% for result in result %}
  <li>{{ result }}</li>
{% else %}
  <li><em>no results</em></li>
{% endfor %}
```

1 files changed, 19 insertions, 0 deletions

diff --git a/testing/tests/loop_else.rs b/testing/tests/loop_else.rs
new file mode 100644
index 0000000..99dfc4b
--- /dev/null
+++ b/testing/tests/loop_else.rs
@@ -0,0 +1,19 @@
+use askama::Template;
+
+#[derive(Template)]
+#[template(
+    source = "{% for v in values %}{{ v }}{% else %}empty{% endfor %}",
+    ext = "txt"
+)]
+struct ForElse<'a> {
+    values: &'a [i32],
+}
+
+#[test]
+fn test_for_else() {
+    let t = ForElse { values: &[1, 2, 3] };
+    assert_eq!(t.render().unwrap(), "123");
+
+    let t = ForElse { values: &[] };
+    assert_eq!(t.render().unwrap(), "empty");
+}