Add the ability to have sub-projects

[zapashcanon]

This would be useful to e.g. work with git submodules.

[lefessan]

Could you be more explicit ? What's the problem ?

[zapashcanon]

Well, that's just a reminder to myself for the problem you told me about:

il y avait cette histoire de faire en sorte que ce soit compositionnel, i.e. qu'on puisse créer un projet drom avec des sous-projets drom

I havn't even tried it yet as I was working on drom init. I'll start working on this one now.

[lefessan]

ah,ok, I was mislead by the reference to git submodules