Add dig feature.

[crayfishx]

Related to #65

#65 was a great suggestion and I felt we could do more with this idea......

From the readme;

By default, when a hash is returned by the HTTP endpoint (eg: JSON) then hiera-http will attempt to lookup the key corresponding with the lookup key. For example, when looking up a key apache::port we would expect the HTTP endpoint to return something like;

{
  "apache::port": 80
}

Returned value would be 80

This behaviour can be overriden by using the options dig and dig_key.

The dig_key option can be used to change the key that is looked up, it also supports a dot-notation for digging values in nested hashes. Special tags can also be used in the dig_key option. Consider the following example output from our HTTP endpoint;

{
  "document": {
    "settings": {
      "apache::port": 80
    }
  }
}

In order to map the lookup to find the correct value, we can interpolate the KEY tag into lookup_key and tell hiera-http to dig into the hash with the following option;

  options:
    dig_key: document.settings.__KEY__

A more complicated example;

{
  "document": {
    "settings": {
      "apache": {
        "port": 80
      }
    }
  }
}

Can be looked up with;

  options:
    dig_key: document.settings.__MODULE__.__KEY__

The dig option can be used to disable digging altogether and the entire data hash will be returned with no attempt to resolve a key

[crayfishx]

This PR also adds a distinction between an undefined value and a deliberatly set nil value.

[crayfishx]

Closes #65

[FlorinAsavoaie]

I think this broke some stuff. I didn't get the time to look into this but http_get returns nil for some cases and then when you test if it's a string, it's not... And it tries to hash_dig it, and it can't call has_key? on nil.

[crayfishx]

@FlorinAsavoaie This should be fixed in 3.3.2 just released.