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.

yamllint/conf/default.yml

@@ -31,7 +31,7 @@ rules:
     max-spaces-after: 1
   indentation:
     spaces: 2
-    # indent-sequences: no
+    indent-sequences: yes
   line-length:
     max: 80
   new-line-at-end-of-file: {level: error}

yamllint/rules/indentation.py

@@ -22,7 +22,8 @@ from yamllint.rules.common import is_explicit_key
 
 ID = 'indentation'
 TYPE = 'token'
-CONF = {'spaces': int}
+CONF = {'spaces': int,
+        'indent-sequences': (True, False, 'whatever')}
 
 ROOT, MAP, B_SEQ, F_SEQ, KEY, VAL = range(6)
 
@@ -164,16 +165,21 @@ def check(conf, token, prev, next, context):
                 # yaml.scan()ning this:
                 #     '- lib:\n'
                 #     '  - var\n'
-                if next.start_mark.column == context['stack'][-1].indent:
-                    #   key:
-                    #   - e1
-                    #   - e2
+                if conf['indent-sequences'] is False:
                     indent = context['stack'][-1].indent
-                else:
-                    #   key:
-                    #     - e1
-                    #     - e2
+                elif conf['indent-sequences'] is True:
                     indent = context['stack'][-1].indent + conf['spaces']
+                else:  # 'whatever'
+                    if next.start_mark.column == context['stack'][-1].indent:
+                        #   key:
+                        #   - e1
+                        #   - e2
+                        indent = context['stack'][-1].indent
+                    else:
+                        #   key:
+                        #     - e1
+                        #     - e2
+                        indent = context['stack'][-1].indent + conf['spaces']
             else:
                 #   k:
                 #     value