Add CSS for hanging-indent div to epub.css.

This is for hanging indents with pandoc-citeproc, to be
supported in a later release.

data/epub.css

@@ -22,3 +22,4 @@ span.smallcaps{ font-variant: small-caps; }
 span.underline{ text-decoration: underline; }
 q { quotes: "“" "”" "‘" "’"; }
 div.column{ display: inline-block; vertical-align: top; width: 50%; }
+div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}