Support Tiktok 
Hi! I'm trying to add Tiktok as a Provider
But Tiktok is asking for `client_key` instead of 
`client_id`
https://github.com/simov/grant/blob/101c95ebb105db2839336606912d35fe97ffd569/lib/flow/oauth2.js#L10

Is it possible to add this Provider?


Here is the doc from Tiktok:
https://developers.tiktok.com/doc/login-kit-web

my config:
``` json
   {
    tiktok: {
      enabled: true,
      icon: 'cube',
      key: '',
      secret: '',
      callback: `${baseURL}/tiktok/callback`,
      scope: ['user.info.basic'],
      origin: 'https://open-api.tiktok.com',
      access_url:'https://open-api.tiktok.com/platform/oauth/connect',
      authorize_url:'https://open-api.tiktok.com/platform/oauth/connect',
      state: true,
      oauth:2,
      transport:'session'
    },
    }
```