Test HTML and Markdown outputs

Summary:
This change provides the classes architecture,
and the output tests.

Follow-up changes will provide concrete implementation.

Ref T1678.

Test Plan: Integration tests provided

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1678

Differential Revision: https://devcentral.nasqueron.org/D2503