godoc: bring back highlighting, selections, and alerts

R=adg, rsc
CC=golang-dev
https://golang.org/cl/5695054
diff --git a/doc/style.css b/doc/style.css
index c387705..821c886 100644
--- a/doc/style.css
+++ b/doc/style.css
@@ -13,6 +13,27 @@
 pre .comment {
 	color: #375EAB;
 }
+pre .highlight {
+	background: #FFFF00;
+}
+pre .highlight-comment {
+	background: #FFFF00;
+	color: #375EAB;
+}
+pre .selection {
+	background: #FF9632;
+}
+pre .selection-comment {
+	background: #FF9632;
+	color: #375EAB;
+}
+pre .selection-highlight {
+	background: #FFFF00;
+}
+pre .selection-highlight-comment {
+	background: #FFFF00;
+	color: #375EAB;
+}
 pre .ln {
 	color: #999;
 }
@@ -88,6 +109,10 @@
 	vertical-align: top;
 }
 
+.alert {
+	color: #FF0000;
+}
+
 #heading {
 	float: left;
 	margin: 0 0 10px 0;