godoc: specify HTML page metadata with a JSON blob

This allows HTML pages to specify arbitrary data in a header:

<!--{
        "Title": "The page title",
        ...
}-->

replacing the old style comments:

<!-- title The page title -->

R=gri, rsc, r, bradfitz, dsymonds
CC=golang-dev
https://golang.org/cl/5532093
diff --git a/doc/debugging_with_gdb.html b/doc/debugging_with_gdb.html
index 874c468..d5c1a88 100644
--- a/doc/debugging_with_gdb.html
+++ b/doc/debugging_with_gdb.html
@@ -1,4 +1,6 @@
-<!-- title Debugging Go Code with GDB -->
+<!--{
+	"Title": "Debugging Go Code with GDB"
+}-->
 
 <p><i>
 This applies to the 6g toolchain. Gccgo has native gdb support. Besides this