ui-tree: use render filters to display content

This allows applying filters to files in the repository, for example to
render Markdown or AsciiDoc as HTML.

Signed-off-by: John Keeping <john@keeping.me.uk>
diff --git a/cgit.css b/cgit.css
index 30c480a..1f581ec 100644
--- a/cgit.css
+++ b/cgit.css
@@ -282,6 +282,11 @@
 	border: solid 1px black;
 }
 
+div#cgit iframe {
+	width: 100%;
+	height: 40em;
+}
+
 div#cgit div.error {
 	color: red;
 	font-weight: bold;