Use keyword list to store headers

Support non-unique headers.
Related #37 and #155

- [x] Change env.header to keyword list
- [x] Ensure all middlewares are working correctly
- [x] Add `get_header/2`
- [x] Add `put_header/3`
- [x] Add `put_headers/3`
- [x] Add `delete_header/2`
- [x] Update README (all examples!)
- [x] Update docs
- [x] Add information to migration guide
