Add global "locale" config option and make key-ordering rule locale-aware

Support sorting by locale with strcoll(). Properly handle case and accents.

Note: this is a second implementation, for context see:
https://github.com/adrienverge/yamllint/pull/280
https://github.com/adrienverge/yamllint/issues/285
https://github.com/adrienverge/yamllint/pull/288

yamllint/config.py

@@ -35,6 +35,8 @@ class YamlLintConfig(object):
         self.yaml_files = pathspec.PathSpec.from_lines(
             'gitwildmatch', ['*.yaml', '*.yml', '.yamllint'])
 
+        self.locale = None
+
         if file is not None:
             with open(file) as f:
                 content = f.read()
@@ -111,6 +113,12 @@ class YamlLintConfig(object):
             self.yaml_files = pathspec.PathSpec.from_lines('gitwildmatch',
                                                            conf['yaml-files'])
 
+        if 'locale' in conf:
+            if not isinstance(conf['locale'], str):
+                raise YamlLintConfigError(
+                    'invalid config: locale should be a string')
+            self.locale = conf['locale']
+
     def validate(self):
         for id in self.rules:
             try: