Basic markdown reformat

[simone-viozzi]

I would like some basic markdown format command.

With rules like:

• no more than one \n above a heading (MD012)
• no images inline
• etc...

We could add a subset of the rules defined by markdownlint

This would also increase compatibility with other markdown render engines.

Note: some rules just do not make sense in silverbullet, like: MD022/blanks-around-headings: Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] markdownlint MD022