Add unit test configuration using karma + mocha + chai

[dragonautdev]

As a user of the brunch-with-vuety skeleton, I want to be able to create a project based on it that already includes at least the most basic unit test configurations for my project so I can easily start testing my application.

Ideally, I would like to have Karma as test runner, Mocha as the test spec framework and Chai as an assertion library for better readability.