Allow nonceEnabled and hashEnabled to take single boolean values

[StephanBijzitter]

Summary

Providing a single boolean value to either of these options will now apply the value to each provided policy directive.

Closes slackhq/csp-html-webpack-plugin#98

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [ ] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).

[CLAassistant]

All committers have signed the CLA.

[StephanBijzitter]

I haven't written any new tests as the existing tests already cover this functionality because of my change to the default values.

If wanted, I could add specific test-cases to handle the 4 shorthand values.