Allow mutable borrow of ui.style (#18)

egui/src/ui.rs

@@ -93,6 +93,12 @@ impl Ui {
         &self.style
     }
 
+    /// Mutably borrow internal `Style`.
+    /// Changes apply to this `Ui` and its subsequent children.
+    pub fn style_mut(&mut self) -> &mut Style {
+        &mut self.style
+    }
+
     pub fn set_style(&mut self, style: Style) {
         self.style = style
     }