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
 ```