Continued positioning when creating steps

Contains changes that were also submitted as part of pull https://github.com/slara/generator-impress/pull/1.

This change is meant to save time by automatically positioning new steps. Users will no-longer **have** to open list.json to position each step, saving time should the user be in too much of a hurry to get creative with his or her step positioning. It also allows a user to manually set the position of two steps that creates a trajectory for the following steps.

A potential enhancement to this would be to add this functionality for rotations as well, allowing the user to easily create a spiraling, rolling, or carrousel effect.