Add "autoplaintext" Feature

This addresses #1 by adding an optional dependency ([markdownify][1]). If installed, you are able to specify *only* an HTML template and the plaintext version will be derived from it. Markdownify has a Beautiful Soup dependency, so it isn't exactly lightweight, but it provides a much more useful result than something like django.utils.strip_tags.

[1]: https://pypi.python.org/pypi/markdownify