Its a common use case to need to modify a Chalice generated template
before deployment. Often to inject resources, values, or
configurations that are not supported directly by Chalice. It will
always be the case that something on AWS is not supported by Chalice
directly that a consumer may want to interact with. This PR opens up
an interface to address that gap more easily in Chalice itself.

The package command can now be invoked with a new argument:

  $ chalice package --merge-template extras.json out

This extras.json file should be a JSON formatted file which will be
deep-merged on top of the sam.json that is generated by Chalice.

For a simple example lets assume that we have the default new Chalice
project and that extras.json has the following content:

```
{
    "Resources": {
	"APIHandler": {
	    "Properties": {
		"Environment": {
		    "Variables": {
			"foo": "bar"
		    }
		}
	    }
	}
    }
}
```

The generated template located at out/sam.json will have the this
environment variable injected into it:

```
...
...
    "APIHandler": {
      "Type": "AWS::Serverless::Function",
      "Properties": {
        "Runtime": "python3.6",
        "Handler": "app.app",
        "CodeUri": "./deployment.zip",
        "Tags": {
          "aws-chalice": "version=1.9.1:stage=dev:app=test"
        },
        "Timeout": 60,
        "MemorySize": 128,
        "Role": {
          "Fn::GetAtt": [
            "DefaultRole",
            "Arn"
          ]
        },
        "Environment": {
          "Variables": {
            "foo": "bar"
          }
        }
      }
    },
...
...
```

Obviously this simple example could have been achieved using the
config file for the environment variables. But using the extras.json
file we could have injected a DynamoDB table into the template, and
injected the table name as a REF into the APIHandler's environment
variables.

For the time being the merge functionality is relatively simple. It can
be extended over time as we discover what types of merge extensions
are needed.

* Simplify merger implementation
* Pull multiple post processor handling out of the AppPackager and into
  a composite post processor
* Make DeepMerger injectable into the TemplateMergePostProcessor to
  allow different mergers to be injected in the future
* Remove config file option, instead you must specify the command line
  option
* Add changelog entry
* Add documentation section for merging under the cfn topic