Add get_dtcs

- Required new function obd_mode_command for commands without PIDs
- Required new reading function for single line responses
- Multiple devices can respond to get_dtcs, so the responses are now
  vectors of the previous type - vectors of u8

src/main.rs

@@ -3,5 +3,17 @@ mod obd2;
 fn main() {
     env_logger::init();
     let mut device = obd2::Obd2::default();
+
     println!("VIN: {:?}", device.get_vin());
+
+    let dtcs = device.get_dtcs();
+    println!("DTCs: {:?}", dtcs);
+    if let Ok(dtcs) = dtcs {
+        for (i, response) in dtcs.iter().enumerate() {
+            println!("DTCs from response {}:", i);
+            for dtc in response {
+                println!("  - {}", dtc);
+            }
+        }
+    }
 }