enhancement: add lint rules plugin support

Add plugin support using setuptools (pkg_resources) plugin mechanism to
yamllint to allow users to add their own custom lint rule plugins,
together with an example plugin implementation and test cases.

Signed-off-by: Satoru SATOH <satoru.satoh@gmail.com>

tests/plugins/example/__init__.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+# Copyright (C) 2020 Satoru SATOH
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""yamllint plugin entry point
+"""
+from __future__ import absolute_import
+
+from . import override_comments
+
+
+RULES_MAP = {
+    override_comments.ID: override_comments
+}

tests/plugins/example/override_comments.py

@@ -0,0 +1,63 @@
+#
+# Copyright (C) 2020 Satoru SATOH
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+"""
+Use this rule to override some comments' rules.
+
+.. rubric:: Options
+
+* Use ``forbid`` to control comments. Set to ``true`` to forbid comments
+  completely.
+
+.. rubric:: Examples
+
+#. With ``override-comments: {forbid: true}``
+
+   the following code snippet would **PASS**:
+   ::
+
+    foo: 1
+
+   the following code snippet would **FAIL**:
+   ::
+
+    # baz
+    foo: 1
+
+.. rubric:: Default values (when enabled)
+
+.. code-block:: yaml
+
+rules:
+  override-comments:
+    forbid: False
+
+"""
+from yamllint.linter import LintProblem
+
+
+ID = 'override-comments'
+TYPE = 'comment'
+CONF = {'forbid': bool}
+DEFAULT = {'forbid': False}
+
+
+def check(conf, comment):
+    """Check if comments are found.
+    """
+    if conf['forbid']:
+        yield LintProblem(comment.line_no, comment.column_no,
+                          'forbidden comment')