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))