footer: link back to cgit home page

The footer has always been overrideable using the footer= in cgitrc, so
this won't anger anybody who cares about their footer.
diff --git a/cgit.css b/cgit.css
index 71b0b9b..6888cde 100644
--- a/cgit.css
+++ b/cgit.css
@@ -590,6 +590,13 @@
 	font-size: 80%;
 	color: #ccc;
 }
+div#cgit div.footer a {
+	color: #ccc;
+	text-decoration: none;
+}
+div#cgit div.footer a:hover {
+	text-decoration: underline;
+}
 div#cgit a.branch-deco {
 	color: #000;
 	margin: 0px 0.5em;