[Feature Proposal] Add position option to addUtilities()

[benface]

Another one of my CSS-order-related feature proposals, heh.

I would like to be able to control where the utilities added by some of my plugins end up in the generated CSS in a very general way. Basically would like to be able to choose between BEFORE or AFTER the core plugins' utilities. Right now they're always added AFTER them.

An example of where this would be useful is my tailwindcss-gap plugin. It generates gap-* utilities that set the margin and/or padding properties, and I want it to be possible to override them with the core m-* or p-* utilities. Right now I achieve that by making the gap-* classes components instead of utilities, but that's a hack. Also, things such as the important option don't get applied to components but they should apply to gap-* classes.

addUtilities() already accepts an options object as its second argument, so I imagine a position option that accepts a string ('after' or 'before').