Add @:include and @:embed Directives

[jenshalm]

It's time to add these directives given how easy they are to implement (it's just a merge of 2 ASTs).

Both directives allow to pass attributes to the embedded document via HOCON directive attributes, which can be accessed with standard substitution references.
The only difference is that @:embed also allows to pass text markup in the form of the directive body.