Format markdown with markdown formatter

**Is your feature request related to a problem? Please describe.**
No relate to any problem.
In some editor we can use markdown formatter (like prettier) to format markdown documents.

**Describe the solution you'd like**
Integrate with prettier or another markdown formatter.

**Describe alternatives you've considered**
Copy content to local editor or online formatter to format documents and paste it back.

**Additional context**
Before
After