Fix HTML tags and quotes in meta caption tag

I was watching [this Raymond Hettinger video](http://pyvideo.org/video/1780/transforming-code-into-beautiful-idiomatic-pytho) and noticed an unexpected `"/>` after the video description. As it happened with me (using Pelican and Jinja) I knew it was probably caused by a not escaped double quote. I'm not experienced in Django templates so I googled around and found the appropriate filters to escape and remove HTML tags, but I'm not sure about the `|safe`.

By the way, sorry for my bad English.