ureq multipart post toolkit

a ureq http multipart post toolkit。

fork from multipart

Examples 1

```ignore

use ureq_multipart::MultipartBuilder;

let (contenttype,data) = MultipartBuilder::new() .addfile("test","/home/feiy/Desktop/1.txt").unwrap() .addtext("name","value") .finish().unwrap(); let resp: Value = req .post(&serviceurl) .set("Content-Type", &contenttype) .sendbytes(&data)? .into_json()?

```

Examples 2

```ignore

use ureq_multipart::MultipartRequest;

let resp: Value = req .post(&serviceurl) .sendmultipartfile("name","&/home/feiy/Desktop/1.txt")? .intojson()?

```