Profile redirects for keybase.io/on/:service/:handle

Gittip used to have a useful feature like this for looking people up, and it was pretty helpful.

I realize that the cli has this, but I'm in a chat trying to encourage folks to sign up for keybase, and it would be really nice to say:

> @gabe: speaking of key signing parties, looks like you're not on keybase.io yet ;)
> https://keybase.io/on/twitter/gggg

Start by examining the CLI's existing lookup behavior and the website's handling of keybase.io URLs. Done means that /on/:service/:handle provides the requested profile lookup or redirect behavior for supported services, with the result verified using the example URL.

Written by the indexing model from the issue text.