allow inplace addition to update currval

examples.py

@@ -261,14 +261,14 @@ def with_example20():
 
 
 @example
-def with_example21():
+def with_example21a():
     with ProgressBar(maxval=1, redirect_stdout=True) as progress:
         print('', file=sys.stdout)
         progress.update(0)
 
 
 @example
-def with_example21():
+def with_example21b():
     with ProgressBar(maxval=1, redirect_stderr=True) as progress:
         print('', file=sys.stderr)
         progress.update(0)
@@ -296,6 +296,27 @@ def example23():
             progress.update(i)
 
 
+@example
+def example24():
+    pbar = ProgressBar(widgets=[Percentage(), Bar()], maxval=10).start()
+    for i in range(10):
+        # do something
+        time.sleep(0.001)
+        pbar += 1
+    pbar.finish()
+
+
+@example
+def example25():
+    widgets = ['Test: ', Percentage(), ' ', Bar(marker=RotatingMarker()),
+               ' ', ETA(), ' ', FileTransferSpeed()]
+    pbar = ProgressBar(widgets=widgets, maxval=1000, redirect_stdout=True).start()
+    for i in range(100):
+        # do something
+        pbar += 10
+    pbar.finish()
+
+
 if __name__ == '__main__':
     try:
         for example in examples:

progressbar/__init__.py

@@ -206,6 +206,11 @@ def __enter__(self):
     # an iterator.
     next = __next__
 
+    def __iadd__(self, value):
+        'Updates the ProgressBar by adding a new value.'
+        self.update(self.currval + value)
+        return self
+
     def _env_size(self):
         'Tries to find the term_width from the environment.'