Allow dependencies' features to be enabled from dependent projects

[jdm]

Canonical example - Servo wants to add a debug-mozjs feature to the mozjs project, where the dependency chain looks like servo -> script -> js -> mozjs. The only way to enable features is from the top-level servo project right now, so we need a way to propagate that to subprojects.

[alexcrichton]

This should be possible since #712 landed in that each dependency will have to reexport the feature of its own dependencies. Did that not end up working out?

[jdm]

I'll try! I didn't know that happened.

[jdm]

Nevermind!