sqlite-utils convert can now use callable references, closes #686

This now works:

    echo '{"date": "13th January 2025"}' | sqlite-utils insert dates.db dates -
    sqlite-utils convert dates.db dates date r.parsedate
    sqlite-utils rows dates.db dates

Previously the command would have been:

    sqlite-utils convert dates.db dates date 'r.parsedate(value)'

docs/cli.rst

@@ -1849,7 +1849,19 @@ These recipes can be used in the code passed to ``sqlite-utils convert`` like th
     sqlite-utils convert my.db mytable mycolumn \
       'r.jsonsplit(value)'
 
-To use any of the documented parameters, do this:
+You can also pass the recipe function directly without the ``(value)`` part - sqlite-utils will detect that it is a callable and use it automatically:
+
+.. code-block:: bash
+
+    sqlite-utils convert my.db mytable mycolumn r.parsedate
+
+This shorter syntax works for any callable, including functions from imported modules:
+
+.. code-block:: bash
+
+    sqlite-utils convert my.db mytable mycolumn json.loads --import json
+
+To use any of the documented parameters, use the full function call syntax:
 
 .. code-block:: bash