Support for Generic `Role`

[Eyas]

Supports Role fully as described in http://blog.schema.org/2014/06/introducing-role.html

Every Property can now be a Role.

This is a pretty risky change, I published schema-dts@0.11.0-beta.1 as an example.

Examples of diffs:

-declare type SchemaValue<T> = T | readonly T[];
+declare type SchemaValue<T, TProperty extends string> = T | Role<T, TProperty> | readonly (T | Role<T, TProperty>)[];

-interface RoleLeaf extends RoleBase {
-    "@type": "Role";
-}
-export declare type Role = RoleLeaf | LinkRole | OrganizationRole | PerformanceRole;
+declare type RoleLeaf<TContent, TProperty extends string> = RoleBase & {
+    "@type": "Role";
+} & {
+    [key in TProperty]: TContent;
+};
+export declare type Role<TContent = never, TProperty extends string = never> = RoleLeaf<TContent, TProperty> | LinkRole<TContent, TProperty> | OrganizationRole<TContent, TProperty> | PerformanceRole<TContent, TProperty>;

 interface PersonBase extends ThingBase {
-    "worksFor"?: SchemaValue<Organization | IdReference>;
+    "worksFor"?: SchemaValue<Organization | IdReference, "worksFor">;
 }

For schema-dts-gen, this change only becomes visible once a Role property is present. I might end up regretting this.

Fixes #143