Pasted tables should use at least 3 hyphens in each column of the header row

[GitHub's guide for Markdown tables](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/organizing-information-with-tables#creating-a-table) indicates:

> There must be at least three hyphens in each column of the header row.

But this plugin creates tables with two hyphens between each column of the header row. This behavior prevents strict renderers, such as Go's blackfriday, from detecting columns in tables pasted with this plugin:

[/russross/blackfriday](https://github.com/russross/blackfriday) ([block.go#L815](https://github.com/russross/blackfriday/blob/e96880f42b9343aea6cbfd99693adae0e5fe2b2a/block.go#L815))

Start by reproducing the pasted-table output and locating the code that generates the header-row separators. Change the generated separators so every column has at least three hyphens, then verify that the resulting table is recognized by strict Markdown renderers such as blackfriday.

Scritto dal modello di indicizzazione a partire dal testo della issue.