Added ability to create crossterm with previously created crossterm::Screen

src/backend/crossterm.rs

@@ -23,6 +23,10 @@ impl CrosstermBackend {
         CrosstermBackend::default()
     }
 
+    pub fn with_crossterm(scr: crossterm::Screen) -> CrosstermBackend {
+        CrosstermBackend { screen: scr }
+    }
+
     pub fn screen(&self) -> &crossterm::Screen {
         &self.screen
     }