Add progress bar for datasets downloading and unpacking
IMDB dataset takes a while to download and nothing is printed in the command-line.

A progress bar would be nice.

Documentation: https://nim-lang.org/docs/httpclient.html#progress-reporting

> Progress reporting
> ==================
> 
> You may specify a callback procedure to be called during an HTTP request.
> This callback will be executed every second with information about the
> progress of the HTTP request.
> 
> ```Nim
> import asyncdispatch, httpclient
> 
> proc onProgressChanged(total, progress, speed: BiggestInt) {.async.} =
>   echo("Downloaded ", progress, " of ", total)
>   echo("Current rate: ", speed div 1000, "kb/s")
> 
> proc asyncProc() {.async.} =
>   var client = newAsyncHttpClient()
>   client.onProgressChanged = onProgressChanged
>   discard await client.getContent("http://speedtest-ams2.digitalocean.com/100mb.test")
> 
> waitFor asyncProc()
> ```
> 
> If you would like to remove the callback simply set it to ``nil``.
> 
> ```Nim
>   client.onProgressChanged = nil
> ```
> 
> **Warning:** The ``total`` reported by httpclient may be 0 in some cases.