config: Allow generic types inside lists

For example it's possible to define a conf like:

    rule:
      foo: [str],
      bar: [int, bool, 'magic'],

yamllint/config.py

@@ -157,11 +157,12 @@ def validate_rule_conf(rule, conf):
                     raise YamlLintConfigError(
                         'invalid config: option "%s" of "%s" should be in %s'
                         % (optkey, rule.ID, options[optkey]))
-            # Example: CONF = {option: ['flag1', 'flag2']}
+            # Example: CONF = {option: ['flag1', 'flag2', int]}
-            #          → {option: [flag1]}      → {option: [flag1, flag2]}
+            #          → {option: [flag1]}      → {option: [42, flag1, flag2]}
             elif isinstance(options[optkey], list):
                 if (type(conf[optkey]) is not list or
-                        any(flag not in options[optkey]
+                        any(flag not in options[optkey] and
+                            type(flag) not in options[optkey]
                             for flag in conf[optkey])):
                     raise YamlLintConfigError(
                         ('invalid config: option "%s" of "%s" should only '