Add option to double-newline before header

Per #157, Please add a style option to add a double-newline before every header

(#157 requested keeping the newlines if present, but as this was hard this should do it ok instead)

Example where `-` is what it does now, and `+` is what I want to round-trip the same:

```
 zork

+
 ============
 First header
 ============

 blah

+
 second header
 -------------

 foo
```
