code: function formatResult (result) {
    return [
      '<file name="' + result.filename + '">',
      result.messages.map(formatMessage).join('\n'),
      '</file>'
    ].join('\n')
  }
docs: Format results for a single file.
  
  @param {object} result Results for a single file
  @param {string} result.filename Filename for these results
  @param {object[]} result.messages Warnings/errors for file
  @return {string} XML representation for file and results