Set user_metadata during signup

[mikeslattery]

Usage. This will set user_metadata with city and state values.

    netlifyIdentity.open('signup', { city: 'Orlando', state: 'FL' })

I've written this already. I'm posting here to see if there is interest in me cleaning up my code and submitting a Github PR that will get approved.

(I use this to set roles and wrote a identity-signup function to copy over whitelisted role values to app_metadata.)

I also made it possible to hide the tab header (Sign up|Log in). I did this in the same commit so clicking a "Login" button can't be used to "signup" without values for user_metadata.

https://gitlab.com/mikeslattery/netlify-identity-widget/-/commit/c8edddbdbce99bddb150ebce01650a98e0ff4816

[Nittarab]

I think this is really useful