Add support for app-only bearer tokens
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.