Adding a template_context keyword to allow passing of fields to templates

The AutoIndex class and render_autoindex method now take an extra keyword argument `template_context` whose contents are passed to `render_template`.  This allows custom templates to get extra fields so they can inherit from and work with other templates in the application.

For example, I want my autoindex.html template to import my base.html, which includes its own breadcrumbs, menus, etc. but needs my fields.

(In case you're interested, I use `add_url_rules = False` and call `render_autoindex` explicitly.)

Thanks!