Utility for the flex-grow property

Created by: tompazourek

I think there could be an utility class like f-grow-1 that would correspond to CSS rule flex-grow: 1. (possibly also f-grow-2, f-grow-3, etc.)

I've been really enjoying working with the Bootstrap 4 flexbox utility classes, but I feel this one is still missing and could be very useful for a lot of people to do stuff like this:

https://stackoverflow.com/questions/29467660/flexbox-row-how-to-stretch-children-to-fill-cross-axis

If it's a stupid idea, please let me know why.