Add option for preserving newlines

[rockboynton]

All newlines seem to be removed with kdlfmt currently, would like to have the option to preserve them.

[dougEfresh]

see kdl-org/kdl-rs#145

[ABDsheikho]

@dougEfresh

I don't think the PR you linked is related to this issue.
The PR addresses formatting of multi-line strings, while this issue discuss the removal of empty lines between options.
Something like this:

entry1 {
    option1 {
        set1
        // comment1
    }

    option2 {
        set2
    }

}

after using kdlfmt it become:

entry1 {
    option1 {
        set1
        // comment1
    }
    option2 {
        set2
    }
}

Not to mention some weird behavior of comments.

[dougEfresh]

My main point is formatting is not primarily controlled by this code. It lives in kdl-rs.
The author of kdlfmt uses whatever format options are in kdl-rs.