Nice to have feature: Tabs

Some of markdown based editors has implemented **tabs**. IMHO ths is the next level markdown editing, and doesn't look hard to implement it. See for example how `Docosaurus 2` [solved this problem](https://v1.docusaurus.io/docs/en/doc-markdown#language-specific-code-tabs).

![image](https://user-images.githubusercontent.com/25417015/112354553-798ae300-8cd5-11eb-8126-6540d4478cff.png)

