examples/session: Add code that demonstrates altering session data

examples/session/index.step

@@ -37,4 +37,27 @@ Here is a horse:
 ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠻⠤⣤⣤⣤⡤⠴⠛⠛⠛⠛⠉⠁⠀⠀⠀⠀⠈⠉⢿⣄⣀⣠⣤⡤⠶⠋⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
 ```
 
+Twilight sparkle demands that you [memorize](.) the sacred powers of two!
+
+{{$list := $session.Get "list" | default (list 1)}}
+{{range $list}}
+- {{.}}
+{{end}}
+{{$last := last $list}}
+{{if lt $last 1}}
+graaaah computer explode xP
+{{else}}
+	{{$list = $last | mul 2 | append $list}}
+	{{$session.Set "list" $list}}
+{{end}}
+
+---
+
+{{$counter := $session.Get "counter" | default 0}}
+You have refreshed the page {{$counter}} times.
+{{$counter = add $counter 1}}
+{{$session.Set "counter" $counter}}
+
+{{if gt $counter 100}}Why?{{end}}
+
 [Exit session](logout.step).