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/disable_with_comments.rst

@@ -0,0 +1,75 @@
+Disable with comments
+=====================
+
+Disabling checks for a specific line
+------------------------------------
+
+To prevent yamllint from reporting problems for a specific line, add a directive
+comment (``# yamllint disable-line ...``) on that line, or on the line above.
+For instance:
+
+.. code-block:: yaml
+
+ # 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
+
+ - This line is waaaaaaaaaay too long but yamllint will not report anything about it.  # yamllint disable-line rule:line-length
+   This line will be checked by yamllint.
+
+or:
+
+.. code-block:: yaml
+
+ # The following mapping contains the same key twice,
+ # but I know what I'm doing:
+ key: value 1
+ # yamllint disable-line rule:key-duplicates
+ key: value 2
+
+ # yamllint disable-line rule:line-length
+ - This line is waaaaaaaaaay too long but yamllint will not report anything about it.
+   This line will be checked by yamllint.
+
+It it possible, although not recommend, to disabled **all** rules for a
+specific line:
+
+.. code-block:: yaml
+
+ # yamllint disable-line
+ -  {    all : rules ,are disabled   for this line}
+
+If you need to disable multiple rules, it is allowed to chain rules like this:
+``# yamllint disable-line rule:hyphens rule:commas rule:indentation``.
+
+Disabling checks for all (or part of) the file
+----------------------------------------------
+
+To prevent yamllint from reporting problems for the whoe file, or for a block of
+lines within the file, use ``# yamllint disable ...`` and ``# yamllint enable
+...`` directive comments. For instance:
+
+.. code-block:: yaml
+
+ # yamllint disable rule:colons
+ - Lorem       : ipsum
+   dolor       : sit amet,
+   consectetur : adipiscing elit
+ # yamllint enable rule:colons
+
+ - rest of the document...
+
+It it possible, although not recommend, to disabled **all** rules:
+
+.. code-block:: yaml
+
+ # yamllint disable
+ - Lorem       :
+         ipsum:
+           dolor : [   sit,amet]
+ -         consectetur : adipiscing elit
+ # yamllint enable
+
+If you need to disable multiple rules, it is allowed to chain rules like this:
+``# yamllint disable rule:hyphens rule:commas rule:indentation``.

docs/index.rst

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