Implement grep with pipe

Signed-off-by: Federico Guerinoni <guerinoni.federico@gmail.com>

src/lib.rs

@@ -26,7 +26,7 @@ pub fn grep(c: Config) -> Result<Vec<String>, &'static str> {
         io::stdin().read_to_string(&mut buffer).unwrap_or_default();
     }
 
-    if c.filenames.len() == 1 && c.filenames[0] == "-" {
+    if c.filenames.is_empty() || c.filenames.len() == 1 && c.filenames[0] == "-" {
         let stdin = io::stdin();
         for line in stdin.lock().lines() {
             let item = line.unwrap_or_default();