Add custom_html to StackLead

In order to use StackLead, we need to send some custom html into their email templates so that our bizdev team can see organization level information as well as Salesforce links:

![image](https://cloud.githubusercontent.com/assets/658544/3434116/6883cf78-0086-11e4-9d37-452b651d0051.png)

The syntax I came up with:

```js
{ integrations: { StackLead: { html: '<h1>woo</h1>' }}
```