Support having Root extend from custom trait

[feynmanliang]

Closes #6

---

This change is 

[feynmanliang]

@aishfenton is this project still active?

[aishfenton]

Yes sorry still alive, let me review and incorporate now.

[TugdualSarazin]

It doesn't compile if the parent object is not in the same package.

it should "support parent, and make the root element extend it" in {
    @fromSchemaResource("/simple.json", parent=Some("argus.macros.test.ParentTest"))
    object Schema

    implicitly[Schema.Root <:< argus.macros.test.ParentTest]
}

package argus.macros.test

trait ParentTest

[feynmanliang]

@TugdualSarazin thanks! I'll take a look at this sometime this week.