Add factory function and "group" option

[Turbo87]

This PR adds a create() factory function to the EslintValidationFilter class, which allows us to return something other than just a EslintValidationFilter instance.

The PR also adds a group option which (in combination with a testGenerator option of qunit or mocha) can be used to generate all tests into a single file and test module:

QUnit.module('ESLint | app');

QUnit.test('a.js', function(assert) {
  assert.expect(1);
  assert.ok(false, 'a.js should pass ESLint\n\n1:1 - Unexpected console statement. (no-console)');
});

QUnit.test('b.js', function(assert) {
  assert.expect(1);
  assert.ok(true, 'b.js should pass ESLint');
});

All this should be implemented in a backwards compatible way so that we can release it in a minor version bump.

Closes #92

see also ember-cli/ember-cli-eslint#176

[rwjblue]

We discussed this in slack, and came to the conclusion that we are 👍 here but that we want to extract the logic to a separate package (to make it easier for others to take advantage and to add additional frameworks).

The plan was to land this here, then extract as a second step.