Preserve at most X lines setting

[jgspratt]

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)