Add esplint to prevent new eslint warnings

[wagenet]

It has been suggested that https://github.com/lint-todo/eslint-formatter-todo would be a better choice than esplint. However, ember-template-lint also uses this library. That means that both can't be run in parallel since they both try to write to the same location. A good improvement to lint-todo would be to allow for a configurable output directory. Since this hasn't yet happened, I propose merging this then switching over when lint-todo is updated.

[locks]

@wagenet do you still intend to pursue this work?

[wagenet]

@locks I think the idea is still good, but would have to re-evaluate the approach at this point.

[kategengler]

Is this still relevant?

[wagenet]

I think work like this is worthwhile if we want to burn down our one-off disabled lints.