switch back to master blackfriday; enable footnotes

As discussed on the mailing list.

Note that the master blackfriday does not have support for `HTML_SKIP_SCRIPT`. There seems to be [a discussion](https://github.com/russross/blackfriday/issues/11) about adding this feature, but it's not there yet.

However, I'm not sure it's really required, as we are not accepting user input for content. If an author does want to embed some script in his content then I don't see why hugo would need to strip it out.