Support for JavaScript files, async processing

Doing a tiny prerelease to assess interest! This adds support for the following:

• quaff now returns a Promise, which means it expects to be used in an async environment.
• New file type! quaff now supports .js files. This means any JavaScript file with a default export (module.exports = ...) is supported. This includes JavaScript files that export functions, including async functions! What does this enable? Anything you can do in a JavaScript file. Hit an API! Pull in some GraphQL! Do some crazy pre-processing!
• General clean up of test hooks - moves to nyc from old raw istanbul calls.

Installation with the next tag via npm:

npm install -D quaff@next