Make progress bar styleable with SCSS variables

[larsmyrup]

I'd like to be able to style the progress bar color using scss variables and to change them between toast types.

Detailed Description

Currently the only way (i see) of changing progress bar colors between toast types is to create a custom css class, inject it with the toastClassName option and changed that value, based on type. To be able to use SCSS variables, would be a lot cleaner.

Context

Our desired design has a more opaque toast background and therefore needs a different progressbar color (than the default), to make it visible. The current way of changing the color is not very clean, would be great to have another way.

It would benefit other users also wanting to change the default progress bar color - or change it between different toast types.

Possible Implementation

Did a PR with my suggestions. Very open to other ideas :-)