Add configuration flag to disable splitting keys on '.'

[lost1227]

I recently set up an instance of Kanboard using this plugin to sign in users through Slack. As part of the set-up, I wanted to have users grouped by the id of the workspace associated with their slack account. The Slack userInfo response does include this information under the key https://slack.com/team_id, so I configured the "Groups Key" to be https://slack.com/team_id.

However, this created two issues:

1. The key https://slack.com/team_id contains a '.', causing the GenericOAuth2UserProvider::getKey function to split it up. Essentially, the plugin thought that the key was referring to a member of a sub-object of the response, when it was actually just a literal '.' in the key.
2. Once I fixed 1, I was still getting a crash. Turns out, the plugin expects the data returned from the "Groups Key" to be an array of groups. However, I was just getting a literal string, causing array_unique to throw an error.

My solution is the following:

1. Add a configuration option to disable splitting keys. None of my keys are composite, so I don't need the splitting functionality. So I added the oauth2_split_keys setting, which will disable key splitting when set to '0'.
2. Wrap the $groups in an array if it is not already an array. That way, if the "Groups Key" returns just a single group, it will be automatically wrapped into an array of length 1.