Resume a previously-aborted post download loop (#732)

With this change, Instaloader is capable of resuming a previously-aborted download loop. To do so, it creates a JSON file within the target directory when interrupted, that contains all the necessary information to later resume that operation.

Resuming an interrupted download is supported for most, but not all targets. It is supported for:

- Regular profile posts,
- IGTV posts
- Saved posts,
- Tagged posts,
- Explore posts.

docs/as-module.rst

@@ -226,6 +226,19 @@ Exceptions
 
 .. autoexception:: TooManyRequestsException
 
+Resumable Iterations
+^^^^^^^^^^^^^^^^^^^^
+
+.. versionadded:: 4.5
+
+.. autoclass:: NodeIterator
+   :no-show-inheritance:
+
+.. autoclass:: FrozenNodeIterator
+   :no-show-inheritance:
+
+.. autofunction:: resumable_iteration
+
 ``InstaloaderContext`` (Low-level functions)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -241,4 +254,4 @@ Exceptions
 .. autoclass:: RateController
    :no-show-inheritance:
 
-   .. versionadded:: 4.5
+   .. versionadded:: 4.5