Added add_post_processor_chain function

[hongjoonchew]

Issue

#115

List of Changes Proposed

Adding Post Processor Chain since its not implemented yet.

Testing Evidence

>>> from packerlicious import post_processor, Template
>>> test_template = Template()
>>> file_post = post_processor.Checksum()
>>> file_post2 = post_processor.Artifice(files = ['testfile.test'])
>>> test_template.add_post_processor_chain(file_post,file_post2)
>>> print(test_template.to_json())
{
  "post-processors": [
    [
      {
        "type": "checksum"
      },
      {
        "files": [
          "testfile.test"
        ],
        "type": "artifice"
      }
    ]
  ]
}

[coveralls]

Coverage decreased (-0.05%) to 82.918% when pulling fb90596 on hongjoonchew:master into cc31f39 on mayn:master.