Add objects nesting support.

This commit enables the implicit conversion from keys, named in a
plain order to keys hier. For example:

section "blah" {
	key = value;
}
section foo {
	key = value;
}

is converted to the following object:
section {
	blah {
		key = value;
	}
	foo {
		key = value;
	}
}

Plain definition may be more complex:

section "blah" "foo" {
	key = value;
}
is presented as:

section {
	blah {
		foo {
			key = value;
		}
	}
}

Suggested by: David Sheets