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

wtf/text_widget.go

@@ -85,3 +85,7 @@ func (widget *TextWidget) addView() {
 
 	widget.View = view
 }
+
+func (widget *TextWidget) UpdateRefreshedAt() {
+	widget.RefreshedAt = time.Now()
+}