Support for progress callbacks

It would be nice if there was a way to attach a progress callback to a particular request. I'd suggest something like this:
```nim
let curl = newCurly()
let resp = curl.get(
  "https://example.com/reallybigfile.tar.xz",
  progressCallback = (
    proc(total, progress, speed: int64) =
      discard # This gets called every 1 second
  )
)
```

Related to: #17, possibly?