Programs should take an init type/function as the first argument

This is in line with the way Elm works. Also update examples.

examples/simple/main.go

@@ -21,12 +21,16 @@ type TickMsg struct{}
 
 func main() {
 	// Initialize our program
-	p := tea.NewProgram(Model(5), update, view, []tea.Sub{tick})
+	p := tea.NewProgram(initialize, update, view, []tea.Sub{tick})
 	if err := p.Start(); err != nil {
 		log.Fatal(err)
 	}
 }
 
+func initialize() (tea.Model, tea.Cmd) {
+	return Model(5), nil
+}
+
 // Update is called when messages are recived. The idea is that you inspect
 // the message and update the model (or send back a new one) accordingly. You
 // can also return a commmand, which is a function that peforms I/O and