Added commit mode

The commit mode ensures pictures are not corrupted when Instaloader is
unexpectedly interrupted. In the case that the last picture is corrupted
because of an interruption, Instaloader will redownload the picture.
Since the metadata is the last object saved to disk, we can consider a
post as "committed" if its json metadata file exists and is not
malformed. Instaloader should download any posts which are not
committed. Downside is commit mode requires metadata to be saved.

docs/cli-options.rst

@@ -230,6 +230,12 @@ How to Download
    to ``3``. If a connection fails, it can be manually skipped by hitting
    :kbd:`Control-c`. Set this to ``0`` to retry infinitely.
 
+.. option:: --commit-mode
+
+   Tries to ensure downloaded images avoid corruption in case of unexpected
+   interruption. If the last picture is corrupted, Instaloader will fix the
+   picture the next time it is run.
+
 Miscellaneous Options
 ^^^^^^^^^^^^^^^^^^^^^