Unneeded Underscore Dependency

[DMPS]

Hi,

I am looking into using your library to test my company's Alexa Skill and I am wondering why you rely on underscore?

I have looked through your code and you only seem to use underscore for the following methods:

1. _.extend
2. _.each
3. .isUndefined/.isNull/_.isString/_isEmpty

Each of these methods have easy ES5 (or earlier) solutions. As you alreasy use ES6 all over the place, there seems to be no reason to try to support older node versions.

If you like, I can raise a PR to strip out the underscore dependency.