code: function useMain (mdFile, swagger) {
    const mainOpts = {
      position: argv.position,
      title: argv.title
    }
    opts.yaml ? mainOpts.yaml = true : mainOpts.json = true
  
    fs.readFile(mdFile, { encoding: 'utf8' }, function (err, md) {
      assert.ifError(err)
      const compiled = remark()
        .use(main(swagger, mainOpts))
        .process(md)
  
      const rs = fromString(compiled)
      const ws = argv.w ? fs.createWriteStream(mdFile) : stdout
      pump(rs, ws, handleErr)
    })
  }
docs: read a file and inject a string of swagger (str, str) -> null