Allow disabling yamllint checks using comments

Implement problem report disabling with comments in YAML source, for
instance:

    # The following mapping contains the same key twice,
    # but I know what I'm doing:
    key: value 1
    key: value 2  # yamllint disable-line rule:key-duplicates

or:

    # yamllint disable rule:colons
    - Lorem       : ipsum
      dolor       : sit amet,
      consectetur : adipiscing elit
    # yamllint enable rule:colons

Closes: #8

docs/index.rst

@@ -23,5 +23,6 @@ Table of contents
    quickstart
    configuration
    rules
+   disable_with_comments
    development
    text_editors