Produce normal single *and* single file

[sandrock]

When option --single-file is not specified: HTML files are produced side-by-side with .md files.

When option --single-file is specified: one HTML file is produced at the specified location.

If both effects are desired, then, we need another option.

Add the --side option to enable production of the side-by-side HTML files.

This would allow: mdbook --side --single-file all.html to produce both effects.