Add quoted-strings rule

* taken from https://github.com/adrienverge/yamllint/pull/110 (submitted by @jurajseffer)
* small fixes for generic and multi-line strings
* fixes for comments from @adrienverge

yamllint/rules/quoted_strings.py

@@ -0,0 +1,74 @@
+# -*- coding: utf-8 -*-
+# Copyright (C) 2018 ClearScore
+#
+# 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 forbid any string values that are not quoted.
+You can also enforce the type of the quote used - single or double - using the
+``quote-type`` option.
+
+**Note**: Multi-line strings (with ``|`` or ``>``) will not be checked.
+
+.. rubric:: Examples
+
+#. With ``quoted-strings: {quote-type: any}``
+
+   the following code snippet would **PASS**:
+   ::
+
+    foo: "bar"
+    bar: 'foo'
+    number: 123
+    boolean: true
+
+   the following code snippet would **FAIL**:
+   ::
+
+    foo: bar
+"""
+
+import yaml
+
+from yamllint.linter import LintProblem
+
+ID = 'quoted-strings'
+TYPE = 'token'
+CONF = {'quote-type': ('any', 'single', 'double')}
+
+
+def check(conf, token, prev, next, nextnext, context):
+    quote_type = conf['quote-type']
+
+    if prev and isinstance(prev, yaml.tokens.TagToken):
+        if prev.value[1] != "str":
+            # we ignore generic strings, e.g. somestring: !!str testtest
+            return
+
+    if isinstance(token, yaml.tokens.ScalarToken):
+        if isinstance(prev, yaml.tokens.ValueToken) or \
+           isinstance(prev, yaml.tokens.TagToken):
+            if ((not token.plain) and
+                    ((token.style == "|") or (token.style == ">"))):
+                # we ignore multi-line strings
+                return
+
+            if ((quote_type == 'single' and token.style != "'") or
+                    (quote_type == 'double' and token.style != '"') or
+                    (quote_type == 'any' and token.style is None)):
+                yield LintProblem(
+                    token.start_mark.line + 1,
+                    token.start_mark.column + 1,
+                    "string value is not quoted with %s quotes" % (quote_type)
+                )