It's now possible to define JSON Text messages!
Minecraft JSON Text gives you many useful options like hover texts or clickable texts (to open a URL or run a command by clicking the text).
You'll find a JSON Text Reference here: http://wiki.vg/Chat or also here: http://minecraft.gamepedia.com/Commands#Raw_JSON_text.
There are also some Websites to generate JSON Text automatically.
Make sure, that your JSON Text isn't malformed.
How to define? Simple! Just add "JSON:" before the message. Example:
The character '&' will still be parsed as color format.Code (YAML):'JSON:{text:"This is a JSON formatted text!",color:"yellow"}'
Keep in mind, the Prefix won't be added to JSON Texts.

Block Messages 2.1
Sends custom messages (chat, hotbar, title, subtitle, JSON) to players, when they click blocks.