Add/Implement Grommet

[bartekus]

I've reimplement Grommet, I think it's a bit more in line with what's expected so it should be pretty good example or starting point.
However any comments are greatly appreciated.

[bartekus]

Unlike with my previous attempt, this time everything works both in dev and prod.

[leebenson]

Thanks @bartekus. Re: this comment, I'd like to add Grommet as a section to the docs rather than supporting it in the core, because it's still a bit opinionated to have a definitive grommit CSS file appear in the webpack build when it might only appeal to a small percentage of the developer base.

Instead, I'd like to keep styling as 'bare' SASS, LESS and PostCSS and then have sections in the docs on how to implement Grommit, Foundation, Bootstrap, etc.

If you don't mind, I'll use this PR as a basis for the new docs for how to implement.

[bartekus]

@leebenson I absolutely don't mind and in fact if I find a little bit of time I'm going to submit PR to the docs with the step-by-step guide for both Grommet and Bootstrap4.

I also agree with keeping the 'kit' bare and only providing implementation 'how-to' as that works quite nicely with the overall philosophy of the project.