add key_tester example

Add an example which displays each key that is pressed.  This is useful for
testing what different terminals produce when keys are pressed.

termwiz/examples/key_tester.rs

@@ -0,0 +1,27 @@
+extern crate failure;
+extern crate termwiz;
+
+use failure::Error;
+use termwiz::caps::Capabilities;
+use termwiz::input::{InputEvent, KeyCode, KeyEvent, Modifiers};
+use termwiz::terminal::{new_terminal, Blocking, Terminal};
+
+const CTRL_C: KeyEvent = KeyEvent {
+    key: KeyCode::Char('C'),
+    modifiers: Modifiers::CTRL,
+};
+
+fn main() -> Result<(), Error> {
+    let caps = Capabilities::new_from_env()?;
+    let mut terminal = new_terminal(caps)?;
+    terminal.set_raw_mode()?;
+
+    while let Some(event) = terminal.poll_input(Blocking::Wait)? {
+        print!("{:?}\r\n", event);
+        if event == InputEvent::Key(CTRL_C) {
+            break;
+        }
+    }
+
+    Ok(())
+}