fix markdown output for blockquotes
fix: multi-part blockquotes

Now
<<EOF
> 1

> 2
EOF
is equivalent to
<<EOF
> 1
>
> 2
EOF
and they mean a blockquote with 2 paragraphs,

but
<<EOF
> 1

> 2
EOF
means 2 separate blockquotes.
semantics change: multi-part blockquotes
fix multipart blockquotes
remove NL after paragraphs in markdown output
functorise Omd_parser.Default_env

because otherwise there may be messy shared values
(in particular the reference environment)