add a disable handler, so that developers can disable a registered task

[palazzem]

This works together with palazzem/wheelie-recipe#11

Without implementing a really complicated API, a recipe is just a list of tasks. Anyway, a developer may want to disable a registered task that could be a dependency of another task (think about browser-sync task in a watch task). Instead of providing an API that:

• removes the browser-sync task in the registry
• updates the watch task so that browser-sync is no more a dependency

we could simply overwrite the browser-sync task with a noop handler that will not do anything except raising an INFO message (i.e.: "The task 'browser-sync' is disabled")