Add a "remember me" option to SignInUser

I think it's fairly common to have some sort of "remember me?" option on login forms. I think we didn't add this in originally because it doesn't make much sense for Api-only apps. So if you're signing in through an API, then chances are that might be handled on the client side. Though, I could see a mobile app that gives you the option to stay logged in by checking a box as the "remember me" :thinking:

In any case, I think it would be cool to add in by default.