code: async function readFile(path, encoding) {
  	const url = browser.extension.getURL(realpath(path));
  
  	return new Promise((resolve, reject) => {
  		const xhr = new global.XMLHttpRequest;
  		xhr.responseType = encoding == null ? 'arraybuffer' : 'text';
  		xhr.addEventListener('load', () => resolve(xhr.response));
  		xhr.addEventListener('error', reject);
  		xhr.open('GET', url);
  		xhr.send();
  	});
  }
docs: Loads a file included in the extension.
  @param  {string}  path      Absolute path of the file to read.
  @param  {string}  encoding  Optional. Allowed values: 'utf-8'
  @return {any}               [description]