Closes #43. Add CommandRunner module to the app.

CommandRunner allows you to define a terminal command and arguments, run
it on a schedule, and view the output.

Examples:

   ping -3 cisco.com

gcal/widget.go

@@ -35,10 +35,10 @@ func (widget *Widget) Refresh() {
 
 	events, _ := Fetch()
 
+	widget.UpdateRefreshedAt()
 	widget.View.Clear()
-	fmt.Fprintf(widget.View, "%s", widget.contentFrom(events))
 
-	widget.RefreshedAt = time.Now()
+	fmt.Fprintf(widget.View, "%s", widget.contentFrom(events))
 }
 
 /* -------------------- Unexported Functions -------------------- */