Remove entire subscription model

It was a valiant effort, and the implementation was solid and
dependable, but at the end of the day we can achieve the same
functionality in a much simpler fashion with commands, especially
because Go is not held to the same restrictions as Elm.

examples/pager/main.go

@@ -18,7 +18,11 @@ func main() {
 
 	boba.AltScreen()
 	defer boba.ExitAltScreen()
-	if err := pager.NewProgram(string(content)).Start(); err != nil {
+	if err := boba.NewProgram(
+		pager.Init(string(content)),
+		pager.Update,
+		pager.View,
+	).Start(); err != nil {
 		fmt.Println("could not run program:", err)
 		os.Exit(1)
 	}