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This required three changes:

(1) Tree.readme() must return the readme file's name along with the content
because Pypeline guesses the markup format from the filename

(2) We must expand the matched readmes to have any (or no) extension, so
that we actually notice markdown files

(3) Adding Pypeline functionality itself, through g.pypeline_markup

Signed-off-by: Wolf wolf@geek.net