Support loading files relative to project root directory

[Mike Tunnicliffe]

Commonly we store configuration files in the project root (or a subdirectory of it like "config").
It is desirable to be able to specify:

manager.load(file: "config.json", relativeFrom: .project)

Additionally, it would be good to have this continue to work if the executable is not located in .build/debug or .build/release, for example, in some other part of the project tree. Therefore the project root should be where the Package.swift resides.

[Mike Tunnicliffe]

I have provided a PR #11 that implement both this and #13

[Youming Lin]

Closed by PR #16.