Update discount submodule
Refactor Discount.Markdown to properly parse documents with quotes

A very slow approach here is to store the Markdown document to a temporary file
and then parse this document directly by the wrapped binary.

So this is not for performance but stability. Keep that in mind!
Adjust tests (check quoted text)
Add test for concurrency performance
Add assertion for quotes in code blocks
Add support for command line options

Must be a list of strings currently.