Optional dependency?

[yunxing]

dependency-env reads "dependencies" field in package.json and reads their exposed env variables.

However, sometimes I have the need to express intention of "if a package exists, do this. If a package is not installed, do that".

Maybe we can have another field called "optDependencies" ? dependency-env can check if a package specified by optDependencies exists and only reads the env variables if it does.