Add support to reload firebase user

[imtoori]

currentUser() returns a cached instance of user. Using reload we can get a fresh instance

[collinjackson]

It seems like developers would want the data of the currentUser() object to change after reload() completes, rather than having to call currentUser() again to get the new data. Perhaps rather than being immutable, currentUser should be a singleton that can be mutated by reload(), as well as the other sign in methods. What do you think?

[imtoori]

That is a clever suggestion, but I took a look at android sdk implementation and it works in that way. https://stackoverflow.com/questions/41488726/get-updated-user-from-firebase-authentication (the first answer)
Also iOS sdk works like that https://stackoverflow.com/questions/47126463/auth-auth-currentuser-reload-doesnt-refresh-currentuser-isemailverified (the first answer again)

[collinjackson]

OK sounds good, let's match the Android and iOS functionality.