There should be a "generic" way to post a proof on any service/website

There are a lot of places where people should be able to post proofs, but it will take a long time to get them all. As a result there should be a generic way to post a proof for any service or website. This has two parts:
1. A proof schema, posted by a website/service and signed using the keybase profile associated with the website. This will contain: (a) the service name, (b) an XPath line for parsing proof pages (to get the text of the proof itself out of the page, eg. from a DIV with a particular ID)
2. A url which contains the proof

So let's say I run a web forum at www.mycoolforums.com and I want my users to be able to post a proof to their user page. I have proven that I own www.mycoolforums.com with my own keybase profile, and post a keybase proof schema at www.mycoolforums.com/.keybase-proof-schema (or something similar - a standard URL just like the proof URL for owning your own website), which will tell Keybase how to parse a proof URL.

Then the user can post their proof to their user page - and then run `keybase prove url` which asks them simply for the url of the proof. When you enter `http://www.mycoolforums.com/users/jlb123', Keybase:
1. Looks for the proof schema on the website and makes sure it's signed properly from the website
2. Loads the proof page and checks the proof for your own keys

This way, any website can post proofs.

Start with the `keybase prove url` entry point described in the issue and review how existing website proofs are discovered and verified. Define the signed schema URL, its service name and XPath, then verify the proof URL against the user's keys; done means a generic website can publish and users can submit its proof URL.

Written by the indexing model from the issue text.