Download a file as parts use basic Range header of HTTP protocol

Run

1. Change targetFileUrl and output variables as you want.
2. Run go run main.go to download. The result should be:

Download file as parts
Path  /photos/853199/pexels-photo-853199.jpeg
Host  images.pexels.com
Size:  23850819
Down load ranges:  [0 4770162 4770163 9540325 9540326 14310488 14310489 19080651 19080652 23850819]
Part: 1 		 Time: 2.645663 seconds 
Part: 4 		 Time: 2.727898 seconds 
Part: 3 		 Time: 2.824624 seconds 
Part: 2 		 Time: 3.145211 seconds 
Part: 0 		 Time: 3.332437 seconds 
Merge files.		 Time: 0.247856 seconds
Output  ./wall_pager.jpg