Adding password provider specific functions

This one may be a little more controversial.

I've added password provider type specific functionality (https://www.firebase.com/docs/security/simple-login-email-password.html). I can extend firebase-login to a firebase-password-login or something to add this but thought I'd see if you think it makes sense to add it to the existing firebase-login component since Firebase includes it in their FirebaseSimpleLogin object.

I also added an attribute & small bit of logic to optionally require a password when creating a user. Firebase does not require passwords in their createUser function.