adds the basic skeleton of our slider

It is a slider made with ascii characters it is bounded between one and
nine has two buttons and when it has focus can be controlled with arrow
keys.
I'd like to get @glawson to chime in here as far as efficiency on [`angular.forEach`](https://docs.angularjs.org/api/ng/function/angular.forEach), but I think this might be a good place to use it. Some have trouble reading `for` loops and using `angular.forEach` may be able to clear up some confusion for those who come after. Plus it looks like it handles objects as well, so it might be a nice standard to have in our project.