Rules: indentation: Add 'check-multi-line-strings' option

This options allows the user to control whether to lint indentation
inside multi-line scalars or not.

When enabled, such YAML source will be detected as a problem:

    - C code: void main() {
                  printf("foo");
              }

whereas this one would not:

    - C code: void main() {
              printf("foo");
              }

yamllint/conf/default.yml

@@ -32,6 +32,7 @@ rules:
   indentation:
     spaces: 2
     indent-sequences: yes
+    check-multi-line-strings: no
   line-length:
     max: 80
   new-line-at-end-of-file: {level: error}

yamllint/rules/indentation.py

@@ -23,7 +23,8 @@ from yamllint.rules.common import is_explicit_key
 ID = 'indentation'
 TYPE = 'token'
 CONF = {'spaces': int,
-        'indent-sequences': (True, False, 'whatever')}
+        'indent-sequences': (True, False, 'whatever'),
+        'check-multi-line-strings': bool}
 
 ROOT, MAP, B_SEQ, F_SEQ, KEY, VAL = range(6)
 
@@ -95,7 +96,11 @@ def check_scalar_indentation(conf, token, context):
         if token.start_mark.buffer[line_start + indent] == '\n':
             continue
 
-        if indent != expected_indent:
+        if indent < expected_indent:
+            yield LintProblem(line_no, indent + 1,
+                              ('wrong indentation: expected at least %d but '
+                               'found %d') % (expected_indent, indent))
+        elif conf['check-multi-line-strings'] and indent > expected_indent:
             yield LintProblem(line_no, indent + 1,
                               'wrong indentation: expected %d but found %d' %
                               (expected_indent, indent))