Compatibility with markdownlint

[Sergej Martynov]

The TODO.md file generated by the extension is not compatible with markdownlint. Strict checking requires that there are no spaces between heading levels, and in TODO.md, after the first level heading (project name), there are third level headings (column names).

I currently solve this problem by manually adding an empty second level heading before the first column heading, but this is a temporary workaround. It would be nice if the column headings were on the second level instead of the third.