GitHub proof for Organizations? More than one GitHub proof per Keybase.io profile?

Wondering whether these should be features?

I did not really want to overwrite my previously-verified GitHub proof for "PublicStartup" just because it has been converted to an "Organization" account. My "PublicStartup" keybase proof is still live:

https://gist.github.com/PublicStartup/2701001506ca9efc8e2d

If the "keybase prove github" command had allowed another GitHub account then this would not have been necessary and my keybase.io profile would now show both my organization and my personal GitHub accounts both verified.

```
$ keybase prove github JasonCoombs
You already have a proof for publicstartup@github; overwrite? [y/N] y
Please publicly post the following Gist, and name it
keybase.md

### Keybase proof

I hereby claim:

* I am jasoncoombs on github.
* I am jasoncoombsceo (https://keybase.io/jasoncoombsceo) on keybase.
```

Start with the issue's `keybase prove github` example and review how GitHub proofs are currently represented and overwritten. Define how organization accounts and multiple GitHub proofs should coexist while preserving existing proofs; done means both the organization and personal accounts can be verified and shown on one profile.

Written by the indexing model from the issue text.