pb: add reset_start_time option (#96)

* Added reset_start_time

* Added docs

* Update pb.rs

src/pb.rs

@@ -310,6 +310,11 @@ impl<T: Write> ProgressBar<T> {
         self.add(1)
     }
 
+    /// Resets the start time to now
+    pub fn reset_start_time(&mut self) {
+        self.start_time = SteadyTime::now();
+    }
+
     fn draw(&mut self) {
         let now = SteadyTime::now();
         if let Some(mrr) = self.max_refresh_rate {