Acquire tokens using authorization code grant flow

Get, store and refresh OAuth tokens using `client_id` and `client_secret` and the [authorization code grant flow](https://dev.twitch.tv/docs/authentication/getting-tokens-oauth#authorization-code-grant-flow).
Before this issue can be developed, these need to be addressed:
- [ ] #4
- [ ] Host bot in a cloud server: should automatically store the OAuth token when the twitch authorization server redirects to our `redirect_uri` or redirect to the localhost server when debugging/testing.