Twitter follow idea

When tracking someone, it would be useful to, since I have a Twitter identity proven that `keybase` knows about, tell me whether the Twitter username of the person I'm trying to track is on my follow list.

For example, I went to track someone whom I follow and whom I know IRL, but didn't recognize their Twitter username immediately. I had to check the Twitter proof directly to satisfy myself that it was who I thought it was.

Minor inconvenience, but Twitter has a social graph, so why not use it?

Relevant API: https://dev.twitter.com/docs/api/1.1/get/users/lookup

```
https://api.twitter.com/1.1/users/lookup.json?screen_name=my_friend

[...]

"following": true
```

Start by tracing the tracking flow and the handling of a proven Twitter identity. Review the referenced Twitter users/lookup endpoint and its following field, then determine how the result should be surfaced when tracking someone. Done means the tracker can indicate whether the person's Twitter username is followed.

Written by the indexing model from the issue text.