gluon-web: add view helper for JSON-encoded values

Can be used for inserting Lua values into inline JS code.

package/gluon-web/luasrc/usr/lib/lua/gluon/web/dispatcher.lua

@@ -87,6 +87,7 @@ local function dispatch(config, http, request)
 		pcdata      = util.pcdata,
 		urlencode   = proto.urlencode,
 		attr        = attr,
+		json        = json.stringify,
 		url         = function(path) return build_url(http, path) end,
 	}, { __index = _G }))