Rules: indentation: Add the 'indent-sequences' option

Using either 'yes', 'no' or 'whatever', the user will be able to choose
whether to force block sequence items to be indented, to force them not
to be indented, or don't care, respectively.

tests/rules/test_comments_indentation.py

@@ -115,7 +115,7 @@ class CommentsIndentationTestCase(RuleTestCase):
                    '  # comments\n'
                    '  b: 2\n', conf)
         self.check('---\n'
-                   'mylist:\n'
+                   'my list for today:\n'
                    '  - todo 1\n'
                    '  - todo 2\n'
                    '  # commented for now\n'