Allow hooks file to be parsed as a template

Add a -template command line option that instructs webhook to parse the
hooks files as Go text templates.

Includes a `getenv` template func for retrieving environment variables.

hooks.yaml.tmpl.example

@@ -0,0 +1,28 @@
+- id: webhook
+  execute-command: /home/adnan/redeploy-go-webhook.sh
+  command-working-directory: /home/adnan/go
+  response-message: I got the payload!
+  response-headers:
+  - name: Access-Control-Allow-Origin
+    value: '*'
+  pass-arguments-to-command:
+  - source: payload
+    name: head_commit.id
+  - source: payload
+    name: pusher.name
+  - source: payload
+    name: pusher.email
+  trigger-rule:
+    and:
+    - match:
+        type: payload-hash-sha1
+        secret: "{{ getenv "XXXTEST_SECRET" | js }}"
+        parameter:
+          source: header
+          name: X-Hub-Signature
+    - match:
+        type: value
+        value: refs/heads/master
+        parameter:
+          source: payload
+          name: ref