Access state param in Add-to-Slack response

[aok-solutions]

When using the Add to Slack button, there is an optional state parameter that allows you to send additional information that will be returned upon completion of the request (Slack recommends using it to avoid forgery attacks).

Is there a way to access this parameter? I will try opening a pull request to see if I can expose it in the creating callback so that users can take action based on it's value.

[dblock]

I believe this shows up as a query parameter, so https://github.com/slack-ruby/slack-ruby-bot-server/blob/master/public/scripts/register.js#L34 and https://github.com/slack-ruby/slack-ruby-bot-server/blob/master/lib/slack-ruby-bot-server/api/endpoints/teams_endpoint.rb#L35 could (should) be modified to include it along with code. Maybe you can test that and contribute?

[dblock]

Closed via #95.