test for Text::Markdown::[Mm]arkdown and use the available one

test for Text::Markdown::[Mm]arkdown and use the available one

Markdown is such a splintered mess.. The current debian package provides
only Text::Markdown::Markdown, while all versions of Text::Markdown support
Text::Markdown::markdown, and old versions also support the capitalised version,
while new ones don't.

It's getting to the point where `grep /markdown/i %symbol_table` is the only
sane way to figure out what function to call..