Keeping line breaks in the transformed code

**Is your feature request related to a problem? Please describe.**
I couldn't find an option to keep the line breaks in the transformed code (while having some selected options, like `mangle`, set to true). Whatever option combination I try, the minified code always ends up with line breaks remove. I know that `minify` is minifier and removing line breaks is the first thing most of the people looks for in a minifier but still, there are some cases when not removing line breaks is what user wants.

**Describe the solution you'd like**
I think adding an option to leave the line breaks as is (except of course for some inlining, etc. that will remove the line breaks anyways) is the right way to go.
Would you be open for such change?