About `"Transfer-Encoding": "chunked"` and streaming response
@dom96 

How about adding a sugar to write streaming response with `"Transfer-Encoding": "chunked"` ?

``` nim
import strutils, math

proc sendChunk*(response: jester.Response, data: string): Future[void] =
  let dataLen = len(data)
  let sizeLen = Positive(floor(log2(toFloat(dataLen)) / 4 + 1))
  response.send(toHex(BiggestInt(dataLen), sizeLen) & "\r\n" & data & "\r\n")

proc endChunk*(response: jester.Response): Future[void] =
  response.send("0\r\n\r\n")
```

Test example:

``` nim
routes:
  get "/":
    var headers = newStringTable({
      "Content-Type": "text/plain",
      "Transfer-Encoding": "chunked"
    })
    await response.sendHeaders(Http202, headers)
    await sleepAsync(1000)

    await response.sendChunk("hello")
    await sleepAsync(1000)

    await response.sendChunk(" world")
    await sleepAsync(1000)

    await response.sendChunk("AAAAAAAAAAAAAAAAAAAAA")
    await sleepAsync(1000)

    await response.endChunk()
```
