Add attribute append command

[minamijoyo]

In #8, I added block append command. However it can add only an empty body for simplicity. Of course, that's not make sense.
In this PR, I also added attribute append command, which is a missing parts for appending a new block in a real use-case.

It allows us to append a new attribute to a given address.
If a matched block not found, nothing happens.
If the given attribute already exists, it returns an error.
If a newline flag is true, it also appends a newline before the new attribute.

$ go run main.go attribute append --help
Append a new attribute at a given address

Arguments:
  ADDRESS          An address of attribute to append.
  VALUE            A new value of attribute.
                   The value is set literally, even if references or expressions.
                   Thus, if you want to set a string literal "hoge", be sure to
                   escape double quotes so that they are not discarded by your shell.
                   e.g.) hcledit attribute append aaa.bbb.ccc '"hoge"'

Usage:
  hcledit attribute append <ADDRESS> <VALUE> [flags]

Flags:
  -h, --help      help for append
      --newline   Append a new line before a new attribute

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

$ cat tmp/attr.hcl | go run main.go attribute append resource.foo.bar.nested.attr3 '"val3"' --newline
resource "foo" "bar" {
  attr1 = "val1"
  nested {
    attr2 = "val2"

    attr3 = "val3"
  }
}