Add options to disable importing CSS

[simonihmig]

In case you want to write some custom CSS it makes more sense to not import the default one, instead of having to override everything. This adds some options (for the app's ember-cli-build.js) to opt out of importing the default CSS.

BTW, removed the use of let in index.js, because that is only available with node 4+.

[simonihmig]

You are welcome! :)

I actually have no problem with requiring node 4, and using let on node-land, but then you should update the requirements in your package.json as well: https://github.com/lifegadget/ui-slider/blob/master/package.json#L16

[yankeeinlondon]

Oh that's a good point 👍