Use mozilla polyfill to enable promise-based API interop with Chrome

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Having some trouble getting this into the build process for extension-boilerplate: https://github.com/mozilla/webextension-polyfill.

Tried manually adding the built polyfill file to extension-boilerplate src and referencing in the manifest.json, but its not making its way into the build/ when building.

Would be a great feature to have. Am currently dealing with a nightmare trying to use the cookies.get() API and support callbacks in chrome and promises in firefox.