Current behavior

Diagram types added for #771 are visible in Boostnote, but when exporting HTML or PDF the diagrams are not rendered.

Expected behavior

The plantuml, mermaid, and other diagram types should be rendered and included in the HTML and PDF exported files.

Steps to reproduce

1. Type up any diagram description until it appears correctly in the preview.
2. File > Export as > HTML|PDF
3. View the HTML or PDF file in a separate application.

Environment

• Version : 0.12.1
• OS Version and name : macOS 10.14.6