Misuse of URLs. Please use URI instead.

When I check a Keybase ID or track it, with the CLI client like this:

```
keybase id github://aef
keybase track twitter://someone
```

The double slash "//" and the general syntax gives the impression that I have to write some kind of URL here. A URL is a kind of URI that describes a resource by its location and is mainly differentiated from other URIs by the aforementioned double slashes after the scheme part. The only problem is, that in this case, no location is described, but just the name of the resource, in this case a user's name.

I think the intention of having a kind of URI here is obvious and a it is indeed a good idea. But I would rather prefer to actually having a generic URI than having a fake URL. I would suggest using something like the following:

```
keybase id github:aef
keybase track twitter:someone
```

For more information see: https://en.wikipedia.org/wiki/URI_scheme

The issue concerns the `keybase id` and `keybase track` CLI commands and their github/twitter identifier syntax. Start at those command entry points and determine the scope of changing `scheme://name` to `scheme:name`; done means the proposed URI syntax is consistently supported.

Written by the indexing model from the issue text.