###  Summary

Describe the goal of this PR. Mention any related Issue numbers.

### Requirements (place an `x` in each `[ ]`)

* [ ] I've read and understood the [Contributing Guidelines](https://github.com/slackhq/csp-html-webpack-plugin/blob/master/.github/CONTRIBUTING.md) and have done my best effort to follow them.
* [ ] I've read and agree to the [Code of Conduct](https://slackhq.github.io/code-of-conduct).
* [ ] I've written tests to cover the new code and functionality included in this PR.
* [ ] I've read, agree to, and signed the [Contributor License Agreement (CLA)](https://cla-assistant.io/slackhq/hack-json-schema).