Pass Slack OAuth redirect_uri query params when present

[sundeepgupta]

The use case is a user issues a slash command, but we can't process their slash command because we don't have their access token yet. So we save the command in our server and respond with an auth link. The user goes through the auth flow and now that we have a token, we wan't to process their original command.

Before Botkit, I was doing this be adding a command=99 query param onto the redirect_uri and I'd check for it when Slack POSTed back with the user's token. It worked nicely.

With Botkit, It looks like I'd have to manually process the POST request. Perhaps this could be passed into the create_user event?

[sundeepgupta]

@peterswimm any thoughts on this? Our app needs a solution to handle oauth flows in different ways, right now I'll have to use a hard fork. Wondering if you see the value in this. I'd imagine all Slack apps would want to behave this way, but maybe it's only me?