[FEATURE] Override Sign up button to open URL

[eoinzy]

Thanks for making this package. I really like it. I have a small suggestion to make it even better!

Is your feature request related to a problem? Please describe.
I have a problem where my registration is complex and requires many different types of fields, more than just email and password and not just text input. There's a payment and email invite functionality, which may be possible on this package but probably not as clean as I'd like. The additionalSignupFields only allow Text fields so that is not suitable (BTW, it would be cool to allow a dropdown here too!). I tried adding another button in the "children" array but that just appears in the middle of the login boxes, so I'm not sure what this is for.

Describe the solution you'd like
For a solution, I would like to see an option to override the click of the Sign Up button to allow opening my website registration URL. Or alternatively, show a text link under the Login button that just says "Register" or "Sign Up" and we can just pass the URL to this and have it open the given web page.

Thanks