Simplify Watcher application config syntax

[sikanrong]

So currently the watcher configuration line looks rather like this:

config :ecstatic, :watchers, fn() -> PhysicsSim.Watchers.watchers end

And I think it might be much simpler to represent it like this:

config :ecstatic, :watchers, [PhysicsSim.Watchers]

...and then internally have Ecstatic concatenate all of the watchers from each module together (by calling watchers/0 on each)