Implement Display for KeyCode and KeyModifiers (#862)

Partially addresses #792
* Implement Display for KeyCode and KeyModifiers
* Add demo for Display implementation

examples/key-display.rs

@@ -0,0 +1,49 @@
+//! Demonstrates the display format of key events.
+//!
+//! This example demonstrates the display format of key events, which is useful for displaying in
+//! the help section of a terminal application.
+//!
+//! cargo run --example key-display
+
+use std::io;
+
+use crossterm::event::{KeyEventKind, KeyModifiers};
+use crossterm::{
+    event::{read, Event, KeyCode},
+    terminal::{disable_raw_mode, enable_raw_mode},
+};
+
+const HELP: &str = r#"Key display
+ - Press any key to see its display format
+ - Use Esc to quit
+"#;
+
+fn main() -> io::Result<()> {
+    println!("{}", HELP);
+    enable_raw_mode()?;
+    if let Err(e) = print_events() {
+        println!("Error: {:?}\r", e);
+    }
+    disable_raw_mode()?;
+    Ok(())
+}
+
+fn print_events() -> io::Result<()> {
+    loop {
+        let event = read()?;
+        match event {
+            Event::Key(event) if event.kind == KeyEventKind::Press => {
+                print!("Key pressed: ");
+                if event.modifiers != KeyModifiers::NONE {
+                    print!("{}+", event.modifiers);
+                }
+                println!("{}\r", event.code);
+                if event.code == KeyCode::Esc {
+                    break;
+                }
+            }
+            _ => {}
+        }
+    }
+    Ok(())
+}