Support Markdown 3

[loqs]

Summary / How this PR fixes the problem?

Adds support for Orc/discount@b9e867b

• Adds build time detection of libmarkdown >= 3.0.0
• Add vala define MARKDOWN3 based on the above
• Change flags to an integer
• If MARDOWN3 is set pass the address of flags rather than the integer itself to mkd.compile

This PR fixes/implements the following bugs/features:

Fixes #400.