extension for automatic children nesting

[Ducasse]

[Ducasse]

I did an experiment in the BookTester package. The idea is that we have

& atag level 1

some contents is automatically put as child of level1 

&& a tag level2 so it is a child of level1

some contents is automatically put as child of level2 

& another tag level 1 this closes the previous child

[Ducasse]

Check this too

https://github.com/pillar-markup/Microdown/issues/543