Support include of markdown files.

[janvda]

The maintenance of big mark down files could be made easier if we could split them in smaller markdown files and include all those smaller markdown files into the master markdown file.

So I think that adding a feature like:

• https://github.com/camelaissani/markdown-it-include

might be interesting.

It is important that this feature doesn't break the table of contents feature ([[toc]]).
In other words the table of contents feature should also consider the contents of all the included markdown files.