Add support for app-only bearer tokens

[sometimes-i-send-pull-requests]

Previously, twitter-to-sqlite only supported OAuth1 authentication, and the token must be on behalf of a user. However, Twitter also supports application-only bearer tokens, documented here: https://developer.twitter.com/en/docs/authentication/oauth-2-0/bearer-tokens This PR adds support to twitter-to-sqlite for using application-only bearer tokens. To use, the auth.json file just needs to contain a "bearer_token" key instead of "api_key", "api_secret_key", etc.