Tests and update the `multipart` and `formidable` recipes

[tunnckoCore]

There may need more tests, PRs are always welcome.

Update formidable recipe

• [ ] test cases for below
• [ ] when IncomingForm instance is passed to options, you should be able to pass IncomingForm options to that options. For example (pseudo-code)

var form = new IncomingForm()
form.on('file', console.log)

app.use(body({
  IncomingForm: form,
  keepExtensions: true,
  uploadDir: path.join(__dirname, 'uploads')
}))

instead of these two ways of doing it

first

var form = new IncomingForm({
  keepExtensions: true
  // etc
})

second (as in recipe and as in formidable README)

var form = new IncomingForm()
form.keepExtensions = true

[tunnckoCore]

finish form recipe

• need README.md file and text