Use client_id and client_secret as default arguments in #authorize_url

Allows you to use already defined `client_id` and `client_secret` to call `::Authorizations.authorize_url`.

```ruby
Octokit.configure do |c|
  c.client_id = 'id'
  c.client_secret = 'secret'
end

Octokit.authorize_url # => https://github.com/login/oauth/authorize?client_id=id&client_secret=secret
```

See #328.