Enable excerpt generation by both BeautifulSoup & lxml

This pull request is addressing the issue referenced here: https://github.com/EnigmaCurry/blogofile_blog/issues/19 and here: https://groups.google.com/forum/#!topic/blogofile-discuss/7Y7eXWOH29o/discussion

I wasn't able to figure out to add the pull request to the existing issue, so I created a new one.

I tried to follow the coding standards of the rest of the code. Please let me know if something needs to be changed.
Thanks!