More flexible Rate Controller (#716)

Move InstaloaderContext's rate controlling logic into a class
RateController with fine-grained methods to enable easily changing
Instaloader's behavior regarding rate limits.

docs/as-module.rst

@@ -229,5 +229,16 @@ Exceptions
 ``InstaloaderContext`` (Low-level functions)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+``InstaloaderContext``
+""""""""""""""""""""""
+
 .. autoclass:: InstaloaderContext
    :no-show-inheritance:
+
+``RateController``
+""""""""""""""""""
+
+.. autoclass:: RateController
+   :no-show-inheritance:
+
+   .. versionadded:: 4.5