Python-thumbnails

- Add ``THUMBNAIL_FORCE_FORMAT``
- Add ``THUMBNAIL_FALLBACK_FORMAT``
- Change ``THUMBNAIL_DUMMY_URL`` to use keyword arguments in string format

~~~~~
- Drop support for Python 2
- Add django filters for markdown and html

~~~~~
- Add missing call of colormode in engine.create

~~~~~
- Add support for base64 encoded images as source
- Add support for colormode through setting the option ``colormode`` or the setting
``THUMBNAIL_COLORMODE``.

~~~~~
- Add Django templatetag ``get_thumbnail``
- Catch IOError and OSError in PillowEngine.engine_load_image and throw ThumbnailError, which
will be caught in get_thumbnail if ``THUMBNAIL_DEBUG = False``.

~~~~~