Add phone number sign-in

[FluorescentHallucinogen]

Please add phone number sign-in to <login-fire>. Something like:

<login-fire phone user="{{user}}"></login-fire>

[masonlouchart]

Enable Phone Number sign-in for your Firebase project

[sbeleidy]

We will likely just wait for it to be implemented in polymerfire (FirebaseExtended/polymerfire#228). Or maybe make a pull request there.

I'm thinking in terms of UI we'd have a phone number input followed by "or" then email input in the login-fire-form. Thoughts?

[masonlouchart]

Perfect!
Of course, we will add a providers property for the modularity of the login-fire-form element.