Adds possibility to define config not only in project extras but also in dependencies

[MekDrop]

This will let to define config not only in project level but also in also in dependencies.

If there are multiple dependencies, with different node or yarn versions, max versions will be selected (yeah I, know, that would be awesome to have multiple nodejs and/or yarn versions installed, but I think this requires some huge changes... so at least something for now...)

Project level config definitions takes priority. That means if these are found, all configs in dependencies will be ignored.

Accepting this change let others to create libraries that depends on node-composer that could be used in projects. For example, a package that automatically bumps owen version everytime new nodejs is released (similar thing that I tried to achieve with #2 pull request). Or some package that automatically tries to deal with front-end npm assets management.

Note: #10 must be merged first to make sure that all platform tests succeeds.

[mariusbuescher]

I think supporting this feature is kind hard work. Especially how it is currently implemented. It only takes the best version and does not care about BC, so a package could specify the latest version of node.js, yes, but that might break with the code using it.

What happens if several packages define different versions and rely on that version?

Seeing the rapid major version releases on node, this is dangerous for a maintainer and will confuse the users and lead to many bugs, that are not straight forward to debug.