[tasks] Add Loop.stop to gracefully stop a task.

Updated docs will follow shortly.

discord/ext/tasks/__init__.py

@@ -38,6 +38,7 @@ class Loop:
         self._before_loop = None
         self._after_loop = None
         self._is_being_cancelled = False
+        self._stop_next_iteration = False
 
         if self.count is not None and self.count <= 0:
             raise ValueError('count must be greater than 0 or None.')
@@ -73,8 +74,12 @@ class Loop:
                 except self._valid_exception as exc:
                     if not self.reconnect:
                         raise
+                    if self._stop_next_iteration:
+                        return
                     await asyncio.sleep(backoff.delay())
                 else:
+                    if self._stop_next_iteration:
+                        return
                     self._current_loop += 1
                     if self._current_loop == self.count:
                         break
@@ -87,6 +92,7 @@ class Loop:
             await self._call_loop_function('after_loop')
             self._is_being_cancelled = False
             self._current_loop = 0
+            self._stop_next_iteration = False
 
     def __get__(self, obj, objtype):
         if obj is None:
@@ -129,6 +135,17 @@ class Loop:
         self._task = self.loop.create_task(self._loop(*args, **kwargs))
         return self._task
 
+    def stop(self):
+        r"""Gracefully stops the task from running.
+
+        Unlike :meth:`cancel`\, this allows the task to finish its
+        current iteration before gracefully exiting.
+
+        .. versionadded:: 1.2
+        """
+        if self._task and not self._task.done():
+            self._stop_next_iteration = True
+
     def _can_be_cancelled(self):
         return not self._is_being_cancelled and self._task and not self._task.done()