Add ability to override back button

[benbahrenburg]

Hello,

Thank you for a great project. I have a use case that I need a custom back button. I've added it as optional in the create controller. I thought it might be helpful for others.

[ranunez]

@benbahrenburg Thanks this looks good! Make sure the function's documentation gets updated accordingly for this new parameter, and update the README with this new customization option

[benbahrenburg]

@ranunez updated the read me and the code comments. Let me know if there is anything else.