Support for `multipart/form-data`.

[fomkin]

Hi, guys!

I want add this feature to your project. That is my plan:

1. Add multipart/form-data codec to headers package
2. Add FormData type and codec to http protocol package
3. Add new body decoder and encoder like it made here
4. Add withFormData and bodyAsFormData methods to HttpRequestOrResponse

What do you think about it?

[pchlupacek]

@fomkin excellent, sounds totally good to me.

[pchlupacek]

@fomkin not sure if you want still to follow on this one ?