code: function writeFile (outDir, vars, file) {
    return function (done) {
      const fileName = file.path
      const inFile = file.fullPath
      const parentDir = file.parentDir
      const outFile = path.join(outDir, maxstache(removeUnderscore(fileName), vars))
  
      mkdirp(path.join(outDir, maxstache(parentDir, vars)), function (err) {
        if (err) return done(err)
  
        const rs = fs.createReadStream(inFile)
        const ts = maxstacheStream(vars)
        const ws = fs.createWriteStream(outFile)
  
        pump(rs, ts, ws, done)
      })
    }
  }
docs: write a file to a directory str -> stream