Add Support for Custom Spinners

[Gabriel Lugmayr]

Would be great to have support for custom Spinners.
To use like that:
<NextNProgress color="black" height="2" > <MyCustomSpinner></MyCustomSpinner> </NextNProgress>

[Sylvester]

This package simply integrates NProgress with Next.js Router.

You can already do what you want by modifying the CSS of the spinner or by passing some custom options into the NextNProgress to change the template of NProgress.