add an example that diffs two files passed as args

examples/diff.rs

@@ -0,0 +1,21 @@
+extern crate diff;
+
+fn main() {
+    let args = std::env::args().collect::<Vec<_>>();
+
+    if args.len() != 3 {
+        println!("usage: cargo run --example diff <first file> <second file>");
+        std::process::exit(1);
+    }
+
+    let left = std::fs::read_to_string(&args[1]).unwrap();
+    let right = std::fs::read_to_string(&args[2]).unwrap();
+
+    for diff in diff::lines(&left, &right) {
+        match diff {
+            diff::Result::Left(l) => println!("-{}", l),
+            diff::Result::Both(l, _) => println!(" {}", l),
+            diff::Result::Right(r) => println!("+{}", r),
+        }
+    }
+}