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 typically only use `make(...)` when I need to be explicit about the minimum/maximum size... which is fairly rare, usually for buffered channels or when I know ahead of time how many values there will be.