--latest-stamps, like --fast-update, but without the need to keep downloaded files (#1131)

Adds the --latest-stamps command line option, pointing to a file where
the latest time each profile was scraped. On the next run, only posts
newer than that time are downloaded.

Fixes #1122.

README.rst

@@ -86,6 +86,16 @@ To later **update your local copy** of that profiles, you may run
 If ``--fast-update`` is given, Instaloader stops when arriving at the
 first already-downloaded picture.
 
+Alternatively, you can use ``--latest-stamps`` to have Instaloader store
+the time each profile was last downloaded and only download newer media:
+
+::
+
+    instaloader --latest-stamps -- profile [profile ...]
+
+With this option it's possible to move or delete downloaded media and still keep
+the archive updated.
+
 When updating profiles, Instaloader
 automatically **detects profile name changes** and renames the target directory
 accordingly.