Implement iterator for context.

src/context/global_settings.rs

@@ -0,0 +1,16 @@
+#[derive(Debug)]
+pub struct GlobalSettings<'s> {
+    placeholder: Option<&'s str>,
+}
+
+impl<'s> GlobalSettings<'s> {
+    pub fn new() -> Self {
+        GlobalSettings { placeholder: None }
+    }
+}
+
+impl<'s> Default for GlobalSettings<'s> {
+    fn default() -> Self {
+        GlobalSettings::new()
+    }
+}