CLI unable to resolve identity proven by multiple Keybase users

**Describe the bug**

I have two Keybase users, [`kg4zow`](https://keybase.io/kg4zow) and [`jms1`](https://keybase.io/jms1). Both have "proven" ownership of my Github account. I did this by combining both proof statements into a [single gist](https://gist.github.com/kg4zow/9ad2650ff8e39bee4da4).

In the GUI client, people are able to find me by my Github username, by selecting "A GitHub user" and entering `kg4zow`. However, if they try to find me using the command line client. they get an error message.

**To Reproduce**

```
$ keybase id kg4zow@github
▶ ERROR In resolving 'kg4zow@github': Identify is ambiguous
```

Also, the error message is incorrect, it should say "**Identity** is ambiguous", but that's minor.

**Expected behavior**

Either a message saying "multiple Keybase users have proven this identity" with a list of the usernames, or the full `keybase id` output for both users, with some kind of separator line between the users to make it obvious where each one starts. Whatever makes sense.

**Additional numbers**

5, 23, and 42. And π, because pie is tasty.

And @chrisnojima's favourite number is 345567. 😎

Start by reproducing the issue with `keybase id kg4zow@github` and trace the CLI's identity-resolution path. The change is complete when multiple matching Keybase users are reported with clear separation or a useful message, and the error says “Identity is ambiguous” rather than “Identify is ambiguous”.

Written by the indexing model from the issue text.