Add a setting (kitty_mod) to change the modifier used by all the default kitty shortcuts, globally

See #487

preprocess-readme.py

@@ -16,6 +16,7 @@ defns = defaultdict(list)
 for line in open('kitty/kitty.conf'):
     if line.startswith('map '):
         _, sc, name = line.split(maxsplit=2)
+        sc = sc.replace('kitty_mod', 'ctrl+shift')
         name = name.rstrip().replace(' ', '_').replace('-', '_').replace('___', '_').replace('__', '_').strip('_')
         defns[name].append('`' + sc.replace('>', ' → ') + '`')