Header followed by paragraph without blank line does not split correctly

README.bigb

= Asdf

== Qwer
zxcv

then:

ourbigbook --split-headers README.bigb

leads to out/html/split.html that contains the Qwer header, and no qwer.html output.

This construct should just be forbidden by linting instead forcing the preferred:

= Asdf

== Qwer

zxcv

Similar problem with preceeding paragraph:

README.bigb

= Asdf

zxcv
== Qwer

The root failure case in both cases is that the header goes inside the paragraph.

Hmm, perhaps that is not a bad behaviour... OK so going back a bit further, the problem is the outcome of:on such cases, which leads to errors.