bug fix: allow code block lines with a single > followed by no space

Insisting on "> " at the start of a code block line meant that
something like

<<dia#diagram=blah>>

> foo = bar
>
> blah = foo

resulted in an error 'foo not in scope'!  The code block was getting
split in the middle into two separate blocks, and hence 'foo' was
being discarded.