Expand environment variable defined in defaults.json

[Benjamin-Tan]

This is to allow defaults.json to reference an environment variable.

[cottsay]

I think this would constitute a breaking change as-is, and would behave differently on Windows where environment variables are denoted with % rather than $. That would imply that defaults files which use this mechanism wouldn't be cross-platform anymore.

Even though this improves the flexibility of defaults files, I think it could hinder debugging problems substantially.

[Benjamin-Tan]

To have cross-platform compatibility, how about having a helper function to support for both $ and %?

It could also have the additional check on the syntax whether any environment variable reference is not defined.