Add feature to generate Markdown table

[ileasile]

I'd like to have writeMarkdown() method that would build markdown table from the simple table. BTW, it could even support some cell formatting

[koperagen]

We can evolve existing df.print to generate valid markdown. It already almost works

[Jolanrensen]

One thing to note here is that this is already possible in newer notebooks with the interactive/native tables :)