Forked and modified...package

[jwiese-ms-zz]

Tony, I forked your project (love the look) here: https://github.com/jwiese-ms/tab_bar_animation

I've modified it to make it generic and more customizable. I was thinking of turning it into a package if that would be ok with you (wasn't sure if you were planning to do that). The last thing I need to do is implement the callback to allow the user to implement their own navigation on tap as well as fix the one issue I have of correctly positioning the 'FAB' on initial creation. Would love you to take a look and give any feedback since you had the original idea. I'm also happy to do a PR if you'd like to incorporate the changes.