A framework for translating themes as been added, which includes:
1. Use of Jinja's `i18n` plugin for translating phrases in templates (not page content).
2. A French translation of the built-in themes (contributions in other languages are welcome).
3. A new `theme.locale` option to define the locale/language of the site.
4. The search plugin uses the value of `theme.locale` as its default 'lang'.
5. Third party themes may chose to use the framework (use is optional).
6. A documented workflow for translating MkDocs built-in themes.
7. Custom tooling for use by translators and theme devs.
The dependencies are not installed by default and are not needed to use MkDocs without translations. However, for anyone who wants to make use of the features, `pip install mkdocs[i18n]` will install all necessary dependencies.
Relates to #211.
