added freezeMiddleware as well as docs for it

[jakebball]

Added a freezeMiddleware built in to freeze the reducer state table to enforce immutability rules. Previously impossible per issue here https://github.com/Roblox/rodux/issues/16 but as mentioned with the addition of table.clone it was trivial to implement.

Very small note:

Added the lua tag to the thunk middleware example on line 271 of of the api reference.