Support for Dart Sass & Jest

• BREAKING: Update API for runSass, which now accepts two arguments: a
  sassOptions object and a trueOptions object.
• BREAKING: Drop support for node < 8
• Add docs and testing for usage with Jest
  #135
• Add sass option to runSass for passing a different Sass implementation
  than node-sass #137
• Remove node-sass from peerDependencies
• Fix deprecated use of assert.fail
  #138
• Update dev dependencies