When no templates are specified always execute a command

Creates a nil renderer when there are no templates provided. Per the README:

> If no templates are provided to a watcher then the command will always be executed.

This satisfies the check:

https://github.com/BlueDragonX/sentinel/blob/master/watcher.go#L85

    if watcher.renderer != nil {

and seems to behave in the way originally intended.