Support of U2F for authentication

[ghost]

GitHub announced it some weeks ago and as they state in the post:

GitHub encourages developers to build U2F support into their own applications as well, enabling authentication with simple user experience and strong security using public key cryptography.

Therefore I think it would be nice if wallabag could support U2F, too. After a quick research I found a PHP library, which propably could be used. You can find more information about U2F here :)

[j0k3r]

This is the part I don't like:

In order to take advantage of the security improvements provided by U2F, you'll need to purchase a hardware key.

But anyway, this is something we should add in a later release.