Downloaders should be predefined inside the distribution package

As shown in the README, in `put-cli.json`

```
{
  "downloader": "wget -O $1/$2 \"$3\""
}
```

OR

```
{
  "downloader": "aria2c -x 4 -s 4 -c -d $1 -o $2 \"$3\""
}
```

these downloaders should be inside the source code, and all the user should write is `wget` or `aria2`. We choose the best set of options to use from `wget` and `aria2` predefined in the CLI.
