log: implement logging to file

[rkhomenko]

Hello. First of all I want to thank you for a great tool.

I often use this program from Python using a subprocess.run function. I capture the stdout and stderr streams for error handling. Unfortunately, the progress bar for the command s5cmd cp is also output to the stderr stream, and I would like to capture all the output of the s5cmd and output the progress bar to the console at the same time.

This small PR adds the ability to log to a file, which will allow error handling and progress bar output.