Support various Markdown flavours

[DaveJarvis]

Add support for a variety of Markdown extensions, including:

• GitHub Markdown
• CommonMark
• JIRA

Markdown-Writer-FX offers a drop-down along the icon bar that gives users the ability to choose a different Markdown flavour.

Replicate

1. Open a Markdown document
2. Click the drop-down to select a Markdown flavour

Expected

The preview pane is updated with a re-rendered version of the document, according to the selected Markdown flavour. For example, a GitHub flavour would render [x] as a checkbox.

Actual

No drop-down exists, no code is written to integrate different Markdown flavours.

Technical

See https://github.com/vsch/flexmark-java/issues/92#issuecomment-293631982