Support custom boxes

[damithc]

It's good if we create custom boxes (similar to current tip boxes) with a combination of color and icon.

<box color="#42b0f4" icon=":rocket:">
    text
</box>

[yamgent]

One possible improvement to the original idea, is to add color and icon attributes to the existing tip-box component instead of creating a new box component.

This allows the user to inherit existing styles (e.g. the user may want to use the info tip-box, but still wants to override the icon with rocket)

<tip-box type="info" icon=":rocket:">
    info but with rocket icon
</tip-box>

@damithc would that be good?

[damithc]

Yes, that works.
I'm in favor of using box instead of tip-box though: shorter and more generic.