async function build (options, { load, ...source }, { create }) {
  if (source.mimeType === 'text/markdown') {
    const raw = await load()
    create(createOptions(source, raw, options))
  }
}
build
@param {Object} options
@param {Array} options.remark plugins for remark
@param {Array} options.rehype plugins for rehype
@param {Object} buildOptions
@param {Object} action
@param {Function} action.create