Can now whitelist users by github pubkeys

Addresses issue #46. Users can now whitelist others based on pubkeys from github as follows:

```
[USER] /whitelist github.com/$GITHUB_USER
-> Added github.com/$GITHUB_USER to the whitelist
[USER] /whitelist github.com/$BAD_GITHUB_USER
-> Error adding to whitelist: No github user github.com/$BAD_GITHUB_USER
```
I've kept the check in a1455a8 since the way keys are parsed is a little different. If the line is split and there *aren't* two elements, then the script would crash, which I would consider worse than some redundancy.