fix: fix GFM markdown output

[aloisklink]

GFM output is currently broken (since v13.2.2), which prevents tables from being written to markdown.

For example, trying to use documentation ./__tests__/fixture/html/nest.input.js -f md will throw an Error: Cannot handle unknown node table` due to this GFM only syntax:

documentation/__tests__/fixture/html/nested.input.js

Lines 173 to 179 in 849a083

	/**
	* | Col 1 | Col 2 | Col 3 |
	* |-------|-------|-------|
	* | Dat 1 | Dat 2 | Dat 3 |
	* | Dat 4 | Dat 5 | Dat 6 |
	*/
	var tableObj = {};

This is because as of remark v13.0.0, GFM support was moved to the remark-gfm pacakge.

6b5bc25 fixed parsing GFM, however, it didn't add GFM support for markdown
output.

Fixes: f4a46b1