Introducing data source for PTR record generation

[johannwagner]

This introduces a data source, which can generate a PTR record on the fly with an ip address as parameter. This fixes #93.

Example:

data "powerdns_ptr" "foo" {
    ip_address = "1.2.3.4"
}

resource "powerdns_record" "foobar" {
  zone    = "in-addr.arpa."
  name    = data.powerdns_ptr.foo.ptr_address
  type    = "PTR"
  ttl     = 300
  records = ["www.example.com."]
}

[johannwagner]

I would also like to add documentation, since this is a user-facing feature, but currently there are no instructions on building or using the documentation part.

[baryluk]

We do not need to use append. We know what is the length of ipv4 and ipv6 address (I would not even assert on that, as you can trust net.Parse to work if there was no error), so array can be preallocated with exact size, and reversing done without append. (there is more you can do, but that might be overkill here).

Also in the IPv4 you use make, but in IPv6 you don't.

[baryluk]

LGTM in general, but the looping can be done simpler.