feat: support responsive grid

[thearnica]

Was looking at tremendous work happening at #11 and probably there are too many changes.

Let's keep in mind why we want responsive grid - only because the target application needs it.

So, let's ask yourself a question - and how the target application gonna do it? How it will provide different grid settings for different media points?

The answer is "somehow", and we want to tap-into that somehow. Be a part of it, not replicate in any way.

---

Long story short - embrace css variables and let application "CSS configuration" let us know what to do.

Example.parameters = {
    grid: {
        columns: 'var(--columns)',
        gutter: 'var(--gutter)',
        gap: 'var(--gap)',
    }
}

PS: gutters and gap already works this way. Now columns can use css var.

fixes #5
closes #11