Handle Guesser::Ambiguous during markdown processing

[johnfairh]

Second part of #1343, handle the 'ambiguous lexer' case and pick an arbitrary one when Rouge is (a) running its markdown lexer and (b) invoked by redcarpet in the same kind of context.

I wasn't sure whether you'd prefer the repeated code factored into a Lexer.find_fancy_safe or something.

[jneen]

👍 from me - the author of the markdown file has full control over the choice of language, so choosing the wrong one seems less bad than throwing an exception here.

[pyrmont]

Thanks for this one, too @johnfairh. It may be worth factoring this into the kind of method you suggested but for the moment, this seems as good an approach as any. Cheers!