tree: support for edge metadata

[dominicbarnes]

From @dominicbarnes on November 21, 2016 19:22

Currently, there's no usage of edge values, so there's no way to distinguish between different edges.

For example, in a database seed system I'm building, there's a distinction between "schema" definitions and "data" definitions, ideally I'd like to easily distinguish between these 2 so the schema can be run first.

I will try to handle this cleanly in the plugin itself, but will probably lean on the tree to maybe expose something like this. (ie: Tree#addDependency(file, metadata)) This complicates the current system a lot, since it'd be a breaking change to modify the way Tree#getDependencies() works. (maybe will need to hide behind a flag)

Copied from original issue: makojs/tree#38