Add session var example with variable (#671)

Author: tony

examples/session-environment.json

@@ -1,14 +1,16 @@
 {
   "environment": {
     "EDITOR": "/usr/bin/vim",
-    "HOME": "/tmp/hm",
+    "DJANGO_SETTINGS_MODULE": "my_app.settings.local",
+    "SERVER_PORT": "8009",
   },  
   "windows": [
     {
       "panes": [
         null, 
+	"./manage.py runserver 0.0.0.0:${SERVER_PORT}"
       ], 
-      "window_name": "Blank pane test"
+      "window_name": "Django project"
     }, 
   ], 
   "session_name": "Environment variables test"

examples/session-environment.yaml

@@ -1,10 +1,10 @@
 session_name: Environment variables test
 environment:
   EDITOR: /usr/bin/vim
-  HOME: /tmp/hm
+  DJANGO_SETTINGS_MODULE: my_app.settings.local
+  SERVER_PORT: "8009"
 windows:
-  # Emptiness will simply open a blank pane, if no shell_command_before.
-  # All these are equivalent
-  - window_name: Blank pane test
+  - window_name: Django project
     panes:
-    -
+    -
+    - ./manage.py runserver 0.0.0.0:${SERVER_PORT}