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

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

docs/configuration.rst

@@ -189,3 +189,22 @@ Here is a more complex example:
      ignore: |
        *.ignore-trailing-spaces.yaml
        ascii-art/*
+
+Setting the locale
+------------------
+
+It is possible to set the ``locale`` option globally. This is passed to Python's
+`locale.setlocale
+<https://docs.python.org/3/library/locale.html#locale.setlocale>`_,
+so an empty string ``""`` will use the system default locale, while e.g.
+``"en_US.UTF-8"`` will use that. If unset, the default is ``"C.UTF-8"``.
+
+Currently this only affects the ``key-ordering`` rule. The default will order
+by Unicode code point number, while other locales will sort case and accents
+properly as well.
+
+.. code-block:: yaml
+
+ extends: default
+
+ locale: en_US.UTF-8