Add support for getting nested block

[minamijoyo]

Partial fix for #21

The hcledit block get command now returns not only the top-level block, but also the nested block.

The previous implementation of the block get command didn't support a nested block. However, it's natural to expect returning the nested block as the same as the attribute in the nested block.

$ cat tmp/attr.hcl
resource "foo" "bar" {
  attr1 = "val1"
  nested {
    attr2 = "val2"
  }
}

$ cat tmp/attr.hcl | go run main.go block get resource.foo.bar.nested
nested {
  attr2 = "val2"
}

$ cat tmp/attr.hcl | go run main.go block get resource.foo.bar
resource "foo" "bar" {
  attr1 = "val1"
  nested {
    attr2 = "val2"
  }
}