Request to add raw markdown writing view

Request new feature to switch the editor view modes, such as raw markdown, splited double columns for synchronized scroll preview, and WYSIWYW as now. Similar products: [Hedgedoc]( Github markdown editor, etc.

Reason

For now, it's quite hard to do some writing jobs on the rendered texts. For example:
• append to link text
• prepend to bold text

Providing a pure markdown code editor somewhere will enhance the ability for delicate operation.

Furthermore, it will be easier to add [Latex]( and [diagrams]( support in the future.