fix thread safety of *PADDING*

Running PARSE-STRING-AND-PRINT-TO-STREAM in multiple threads can
interfere for each other.
add support for markdown as an output format

This is useful when the input markdown is converted into parse tree
with 3BMD-GRAMMAR:PARSE-DOC, the parse is munged and is then printed
with 3BMD::PRINT-DOC-TO-STREAM-USING-FORMAT.