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/cli-options.rst

@@ -223,6 +223,29 @@ How to Download
    ``#hashtag`` or the profile name. Defaults to ``{date_utc}_UTC``.
    See :ref:`filename-specification` for a list of supported tokens.
 
+.. option:: --resume-prefix prefix
+
+   For many targets, Instaloader is capable of resuming a previously-aborted
+   download loop.  To do so, it creates a JSON file within the target directory
+   when interrupted.  This option controls the prefix for filenames that are
+   used to save the information to resume an interrupted download.  The default
+   prefix is ``iterator``.
+
+   Resuming an interrupted download is supported for most, but not all targets.
+   JSON files with resume information are always compressed, regardless of
+   :option:`--no-compress-json`.
+
+   This feature is turned off entirely with :option:`--no-resume`.
+
+   .. versionadded:: 4.5
+
+.. option:: --no-resume
+
+   Do not resume a previously-aborted download iteration, and do not save such
+   information when interrupted.
+
+   .. versionadded:: 4.5
+
 .. option:: --user-agent USER_AGENT
 
    User Agent to use for HTTP requests. Per default, Instaloader pretends being