Submodules

Mask is very cool!

The one feature I think would make it much cleaner is the ability for submodules.

Right now we have
```md
## build
...
## test
...
```

where we have all the commands in a single long file.

Ideally you should allow

```md
## [build](./mask/build.md)
## [test](./mask/test.md)
```

Even looks cool on MD.

