soroban basic addition, alpha release

First release of soroban utility script and soroban basic addition
exercise. Reformatting to fit Khan exercise standards is needed, as well
as updates to handle decimal values.

This is my first attempt to contribute both to khan-exercises and to any project via Github, so apologies up front if I'm breaking any normal practices or coding conventions. This is a relatively simple combination of using Graphie to draw a soroban (abacus), Interactive to create movable points to function as soroban beads, and some math to turn bead positions into numeric values.

Among intuitive bead-dragging behavior, grabbing a bead in the bottom (earth) section and dragging it left performs a "pinch", where the top (heaven) and bottom beads are both moved toward the divider bar, and likewise dragging right performs a "flick", where the beads on top and bottom are pushed away from the divider bar.