Allow custom build settings to be defined for dependencies

For example:
---
{
    "name": "example",
    "dependencies": {
        "vibe-d": { "version" : "~>0.9.2", "dflags" : ["-preview=in"] }
    }
}
---

In this example, `-preview=in` will be applied to `vibe-d` and all of its dependencies.
Any $(LINK2 build settings, https://dub.pm/package-format-json.html#build-settings) field will be parsed,
however only `dflags` is taken into account when compiling for now.