Customizable Action stub, replacements

[AkenRoberts]

Allow users to specify their own stub file, and optionally add their own replacements.

Stub customizing should be straight forward (famous last words, I know). Might have an opportunity to port it into Laravel's core also. (Thinking emulating View's file loader + stub namespace, kind of like how vendor views work.)

Replacements could be done in a pipeline mechanism. Interface + small individual classes could define the string to replace, and functionality to do so. And/Or simple closures.