Feature request: Polar and spherical coordinates for positioning of steps

It would be great if it was possible to position the steps by polar coordinates. Makes positioning much easier if you use rotation or slides arranged with relative rotations to each other (e.g. positioning them along a circle).
I would propose new `data-angle` and `data-distance` (polar coordinates - 2D) and `data-angle-primary`, `data-angle-secondary`, `data-distance` (spherical coordinates - 3D) attributes along with the corresponding relative ones.
One could also think of a astronomical positioning system (azimuth, elevation, distance).

The rest should be a relative simple math formula with `math.pi`, `math.sin` and so on.

When I have the time I would be glad to help with the implementation, but at the moment I have no spare time for this.

The issue names no files or tests. Start by reviewing how impress.js positions steps and handles existing relative coordinates, then define support for the proposed polar and spherical attributes and their corresponding relative forms; done means those coordinates position steps as described.

Written by the indexing model from the issue text.