Add mouse_down accessor function

CHANGELOG.markdown

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- `ImGui::mouse_down`
+
 ## [0.0.20] - 2018-08-13
 
 ### Fixed

src/lib.rs

@@ -246,6 +246,10 @@ impl ImGui {
         let io = self.io();
         (io.mouse_delta.x, io.mouse_delta.y)
     }
+    pub fn mouse_down(&self) -> [bool; 5] {
+        let io = self.io();
+        io.mouse_down
+    }
     pub fn set_mouse_down(&mut self, states: [bool; 5]) {
         let io = self.io_mut();
         io.mouse_down = states;