Add journal templating function readme and example simulation

examples/journal-templating/README.md

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+
+# Journal templating
+
+This new templating function allow you to query data from the journal to generate dynamic response.
+
+Use the provided Hoverfly binary, run it in capture mode (which allows request to be passed through and recorded in the journal),
+and also import the simulation `journal-templating.json`.
+
+```commandline
+ hoverfly -capture -import journal-template.json -journal-indexing-key Request.QueryParam.id 
+ curl --proxy localhost:8500 'time.jsontest.com?id=123'
+```
+
+The request is now recorded, and we can switch Hoverfly to simulate mode, and then this time we can call the stateful endpoint which will trigger the simulation:
+```commandline
+ hoverctl mode simulate
+ curl --proxy localhost:8500 'time.jsontest.com/stateful'
+```
+
+You will see that the stateful endpoint response is the time field from the previous response for `localhost:8500 'time.jsontest.com?id=123`
+That's because the simulation uses this templating function:
+
+```json
+
+    "response": {
+      "status": 200,
+      "body": "{{ journal 'Request.QueryParam.id' '123' 'Response' 'jsonpath' '$.time' }}",
+      "encodedBody": false,
+      "templated" : true
+   }
+
+```
+
+What it does is that it look up the journal with this index key `Request.QueryParam.id`, 
+so if any of the previous request has a request query param with key `id` and value `123`, 
+then it will return the response body of that request, and then apply the jsonpath expression `$.time` to extract the time field from the response body.
+
+The index key `Request.QueryParam.id` is just an example, you can specify other request data as index to look up using the same syntax as templating request data:
+https://docs.hoverfly.io/en/latest/pages/keyconcepts/templating/templating.html#getting-data-from-the-request
+
+eg. if you want to look up the journal by the second request path variable, you can use this index key: `Request.Path.[1]`
+
+```commandline
+ hoverfly -journal-indexing-key Request.Path.[1]
+```
+
+You can also specify multiple index keys like this: 
+
+```commandline
+ hoverfly -journal-indexing-key Request.Path.[1] -journal-indexing-key Request.Header.X-Header-Id 
+```
+
+In which case we will create multiple indices for the journal based on the given keys. It allows you to lookup all the past request/response using those keys.

examples/journal-templating/journal-template.json

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+{
+  "data": {
+    "pairs": [
+      {
+        "request": {
+          "method": [
+            {
+              "matcher": "exact",
+              "value": "GET"
+            }],
+          "path": [
+            {
+              "matcher": "exact",
+              "value": "/stateful"
+            }
+          ]
+        },
+        "response": {
+          "status": 200,
+          "body": "{{ journal 'Request.QueryParam.id' '123' 'Response' 'jsonpath' '$.time' }}",
+          "encodedBody": false,
+          "templated" : true
+        }
+      }
+    ],
+    "globalActions": {
+      "delays": []
+    }
+  },
+  "meta": {
+    "schemaVersion": "v5.2",
+    "hoverflyVersion": "v0.10.2",
+    "timeExported": "2017-02-23T12:43:48Z"
+  }
+}