Dj-angles

- Add support for `dj-if`, `dj-elif`, `dj-else` attributes.
- Add `initial_attribute_regex` setting.

- Better handling of spaces and tabs in template tag arguments.

- Handle newlines in template tag arguments.

- Self-closing `dj-block` tag; `<dj-block name='content' />` would translate to `{% block content %}{% endblock content %}`.
- Handle template includes that begin with an underscore; `<dj-partial />` would translate to, in order, either `partial.html` or `_partial.html` depending on which template file was found.

- Use the start tag's `name` for `dj-block` end tag if possible.

- Add `default_mapper` based on [7](https://github.com/adamghill/dj-angles/pull/7) from [nanuxbe](https://github.com/nanuxbe).