Change API to take method name and team id pairs

This completely changes the API for omniauth-github-team-member. Before you set `GITHUB_TEAM_ID` in your env, but now you pass in a hash of method name and team id pairs.

```ruby
Rails.application.config.middleware.use OmniAuth::Builder do
  provider :githubteammember,
    ENV["GITHUB_CLIENT_ID"],
    ENV["GITHUB_CLIENT_SECRET"],
    :scope => "user",
    :teams => {
      "mentors_team_member?" => 426344
    }
end
```

Now wherever I have an omniauth hash I can ask it `omniauth_hash.mentors_team_member?` to find out if the user is a member of the mentors team.

This lets you pass in as many different method name and team id pairs as you want, so you can do different things in different parts of your app based on different team memberships.