Preserve at most X lines setting

I would like to be able to preserve two blank lines (any more than two blank lines should be reduced to two blank lines).

I suggest the following backwards compatible change:

* `retain_line_breaks` (bool) - Retains all line breaks
* `retain_line_breaks_single` (bool) - Retains at most one line break (takes precedence over `retain_line_breaks`)
* `retain_line_breaks_count` (int) - Retains at most this many line breaks (takes precedence over `retain_line_breaks` and `retain_line_breaks_single`)