There is an old feature request for this:#360

But it's unlikely to happen as I've pretty much stopped any new feature development on sql-formatter.

Instead I've incorporated many of the ideas and addressed lots of the shortcomings of this inanother SQL pretty-printing tool that I've been developing. This one pretty much does what you're asking for: if the input SQL is on a single line and it's short enough, the formatter will keep it on a single line.