Prevent to broadcast the code to every tishadow client

* Each snippet has it's own room.
* The room name is the hash of the url minus first two characters.

ex:

[url]
http://localhost:3000/#-IulS2vGk-CxDrV32iuk

[room]
IulS2vGk-CxDrV32iuk

Signed-off-by: Justin Lee <lis186@gmail.com>