Add option to use IdToken instead of AccessToken

[sschadwick]

#119

[pi0]

Default shouldn't be false?

[pi0]

Thanks for the PR. BTW what's your idea if we can generalize this option little more. Like having an option like tokenKey (or something better. I'm not good at naming!) that can be id_token/access_token or anything else.

[sschadwick]

You're absolutely right, I've updated the commit. I like the idea of using tokenKey as the key but it would involve string matching, which is never my first choice. However, if that's how you want to proceed, how does this sound: tokenKey: 'access_token' and tokenKey: 'id_token'?

[pi0]

@sschadwick Yep exactly what i was mean. Something like this:

let token = parsedQuery[this.options.tokenKey]

[sschadwick]

Okay updated again. I used your suggestion but changed tokenKey to token_key to remain consistent with the other config options. Also added access_token as the default.

[pi0]

Thanks for your contribution!