Add support for html5 caption
Merge pull request #443 from joshmcrty/master

Add support for HTML5 captions.
_s: Simplify caption styles to work in HTML5 mode.

Since HTML5 captions removed the additional 10px that HTML4 versions
come with, the `max-width: 98%;` rule made images blurry. This slightly
simplifies caption styles while still providing support for both HTML4
and HTML5 versions.

See #443.