Save content using original filenames (#893)

Add feature from issue #892 (Save content using original filenames)

- The new feature would download posts and save the images/videos with the original filenames.
- The implementation follows the other options for the filename-pattern parameter. As filename is related to a single file and not the whole post, for sidecar nodes the filename is calculated again.
- Using this option media from one post is only grouped together by the file modification timestamp.

docs/basic-usage.rst

@@ -171,6 +171,9 @@ dirname pattern. The following tokens are defined for usage with
 - ``{mediaid}``
    Integer representation of shortcode.
 
+- ``{filename}``
+   Instagram's internal filename.
+
 - ``{date_utc}`` (same as ``{date}``)
    Creation time in UTC timezone.
    `strftime()-style formatting options <https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior>`__