Metadata parsing does not work on Windows

https://github.com/caolan/wmd/blob/master/lib/preprocessors.js uses `\n` as the newline character and thus fails to correctly parse metadata lines on Windows.

Start with lib/preprocessors.js, which the issue identifies as using the wrong newline character for metadata parsing. Check how metadata lines are split and verify the result against Windows line endings; done means metadata parses correctly on Windows without breaking existing newline handling.

Written by the indexing model from the issue text.