Illustrate how to monitor the primary selection. (#15)

Inspired by https://github.com/farseerfc/ydcv-rs/blob/master/src/main.rs#L143

Fixes #14

examples/monitor_primary_selection.rs

@@ -0,0 +1,29 @@
+extern crate x11_clipboard;
+
+use x11_clipboard::Clipboard;
+
+
+fn main() {
+    let clipboard = Clipboard::new().unwrap();
+    let mut last = String::new();
+
+    println!("Waiting for selection...");
+
+    loop {
+        if let Ok(curr) = clipboard.load_wait(
+            clipboard.getter.atoms.primary,
+            clipboard.getter.atoms.utf8_string,
+            clipboard.getter.atoms.property
+        ) {
+            let curr = String::from_utf8_lossy(&curr);
+            let curr = curr
+                .trim_matches('\u{0}')
+                .trim();
+            if !curr.is_empty() && last != curr {
+                last = curr.to_owned();
+                println!("Contents of primary selection: {}", last);
+                println!("Waiting for selection...");
+            }
+        }
+    }
+}