Add verified provider to example app

[umputun]

@Reeywhaar - I have added a new provider to allow self-verified users, i.e. auth with smth like email to deliver confirmation token. This token can be used to login. See #23 for details.

In the demo application, it called "email". Will be nice to have UI support for this provide as you have some time to spend. I think this is as simple as open a box with username and email fields and submit button hits /auth/email/login?user=blah&address=aaa@bbb.com. After the click, it should show another input box asking for confirmation token. The token won't be sent by email, but just printed to stdout. With this token UI should hit /auth/email/login?token=xxxxxx and it will do the usual auth.

[umputun]

thx @Reeywhaar, works fins