support indieauth protocol

[tantek]

It would be great if relmeauth.php was able to support the indieauth protocol as well, for inbound requests to a site, so you could use your own relmeauth.php deployment as an indieauth endpoint for sites like indieweb.org and woodwind.xyz to use your own site's auth server for your domain, instead of depending on indieauth.com

See https://indieweb.org/indieauth-for-login for a start and some details for how to implement.