Add flag Keep spaces in file names

[jgspratt]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.06.16.1
• I've searched the bugtracker for similar feature requests including closed ones

Description

The --restrict-filenames is great except that it results in filenames which many search programs do not search effectively. For example, youtube-dl_test_video_a downloaded from BaW_jenozKc cannot be found with a search term of test video in many file systems.

I propose a --restrict-filenames-allow-spaces option which would result in a file name of youtube-dl test video.

This was opened as a question here: #25962