Adds support for chisting in Slack web format

This PR adds support for creating chists using the Slack web client chat format.
The following screenshot shows how it looks:

![image](https://cloud.githubusercontent.com/assets/43977/5519452/01e93bc8-8933-11e4-9c47-b998b5693db0.png)


![image](https://cloud.githubusercontent.com/assets/43977/5519425/13039e54-8932-11e4-91b3-a290e433158b.png)