Chat Plugins support

Here my first try to add Chat Plugins support.

* All the chat parsing logic has been moved to the [chat directory](https://github.com/superjoe30/mineflayer/tree/05337bce8724bb51511bd0ca11ce9f4782142ad5/lib/plugins/chat).
  * All Chat Plugins inherit from [ChatBase](https://github.com/superjoe30/mineflayer/blob/05337bce8724bb51511bd0ca11ce9f4782142ad5/lib/plugins/chat/base.js) class.
* Included a [plugin implementation example](https://github.com/superjoe30/mineflayer/blob/05337bce8724bb51511bd0ca11ce9f4782142ad5/examples/herochat.js).

The following plugins included by default:

* `vanilla` (default)
* `bukkit`
* `essentials`
* `herochat`
* `vanilla_old` (for minecraft <= 1.6.1)

This PR is related with the following issues: #159, #161 and #163.

What do you think about this approach? Any opinion is welcome. If anyone can test it and report bugs would be wonderful.