Markdown tables not strict about syntax

Markdown cells seem to properly generate tables, even when the markdown syntax is wrong; this causes issues for example, when converting a notebook to markdown and viewing it on something like GitHub.

```
a | b | c | d
--- | --- | -- |
1 | 2 | 3 | 4
```

Produces the following in a Markdown cell:


And gets rendered on Github as:


Start with the malformed Markdown table example in the issue and compare how the Markdown cell renders it with how GitHub renders the converted Markdown. Done means invalid table syntax is not accepted as a valid table, with the notebook-to-Markdown result matching strict Markdown behavior.

Written by the indexing model from the issue text.