Add support for nested structure, e.g. attachments.
Add support for nested structure, e.g. attachments.

> The optional attachments argument should contain a JSON-encoded array of
> attachments.
>
> https://api.slack.com/methods/chat.postMessage

This enables simpler calls like this:

    sc.api_call('chat.postMessage', {'attachments': [{'title': 'hello'}]})

slackapi/python-slackclient,slackapi/python-slackclient,slackapi/python-slackclient,slackhq/python-slackclient