Use Post class attributes in filename-pattern

- Added owner_id and mediaid to Post class properties.
- In case of not downloading stories, the attributes of the Post class
  can now be used in filename-pattern, e.g. {post.owner_id} or {post.mediaid}.

Closes #53.

docs/README.md

@@ -10,6 +10,6 @@ https://instaloader.readthedocs.io/
 The documentation is created with [Sphinx](http://www.sphinx-doc.org/). To build it, use
 
 ```
-pip3 install sphinx
+pip3 install sphinx sphinx-autodoc-typehints
 make html
 ```

docs/basic-usage.rst

@@ -96,7 +96,8 @@ pattern, the token ``{target}`` is replaced by the target name, and
 to the target directory. The default is ``--filename-pattern={date}``.
 The tokens ``{target}`` and ``{profile}`` are replaced like in the
 dirname pattern. Further, the tokens ``{date}`` and ``{shortcode}`` are
-defined.
+defined. Additionally, in case of not downloading stories, the attributes of
+:class:`.Post` can be used, e.g. ``{post.owner_id}`` or ``{post.mediaid}``.
 
 For example, encode the poster's profile name in the filenames with:
 

docs/cli-options.rst

@@ -134,8 +134,11 @@ How to Download
    with ``--dirname-pattern``.  ``{profile}`` is replaced by the profile name,
    ``{target}`` is replaced by the target you specified, i.e.  either ``:feed``,
    ``#hashtag`` or the profile name.  Also, the fields ``{date}`` and
-   ``{shortcode}`` can be specified.  Defaults to ``{date:%Y-%m-%d_%H-%M-%S}``.
-   See :ref:`filename-specification`.
+   ``{shortcode}`` can be specified.  In case of not downloading stories, the
+   attributes of the :class:`.Post` class can be used in addition, e.g.
+   ``{post.owner_id}`` or ``{post.mediaid}``.
+   Defaults to
+   ``{date:%Y-%m-%d_%H-%M-%S}``.  See :ref:`filename-specification`.
 
 .. option:: --user-agent USER_AGENT