Move Sword::Task.definer logic into Sword::Syringe

Such an abstract entity as Task should be tool-agnostic: it should not
lock on Thor compatibility only, it is just an abstract task that has a
description, long description and the invoked code.

If I want to, I should be able to easily change Thor to another tool, be
it optparse, Commander or whatever. For this to happen, now the only
things I need to change are Sword::Syringe and Sword::CLI.

And yes, this commit moves plugins loading from Sword::Syringe.
To be continued.