<caption> is stripped in iframes

If there is a `` tag in an iframe, it is stripped. The output of this can be seen by building the "styleguide" branch with the "0.3" branch at `/styles/typography.less.html` in the "Content Tags" demo.

Source file: https://github.com/bitovi/documentjs/blob/styleguide/demos/typography/content/demo.html#L24

Start with demos/typography/content/demo.html at the referenced line, then reproduce the issue by building the "styleguide" branch with the "0.3" branch. Inspect the Content Tags demo output at /styles/typography.less.html. Done means the tag is preserved when rendered inside an iframe.

Written by the indexing model from the issue text.