use-brace-expansion and spaces

In the [docs/rules](https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/use-brace-expansion.md) it mentions that spaces are not allowed. A developer would also encounter an Ember error upon the code being used in the app (as well as failing tests). However I would argue that this should be caught at the eslint level so it could be fixed before the developer even leaves their editor.

I tried doing this in the existing rule, but ran into a few edge cases where it didn't catch it. When I had the great idea: should this be its own rule or added on to the existing one?

I'm happy to implement it either way, but didn't want to make a judgement call and have it not be inline with the repo. Let me know and I can finish working on it :)