Fixed #6799 - added an `end_text` argument to `truncate_words`/`truncate_html_words`.

This allows customizing the standard "..." end text. Yes, this is technically a
feature sneaking in after the deadline, but I just couldn't bring myself to punt
it again: we already used that excuse for not getting it into 1.1.

Thanks to Adam Fast and Travis Cline for work on this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12431 bcc190cf-cafb-0310-a4f2-bffc1f526a37