Ability to disable handlers

This feature had been in V1, but got lost in the refactoring. Sometimes you may want the ability to disable particular destination or field handlers. The immediate example is moving contrib module support from Migrate Extras to the contrib modules themselves - we don't want to remove support from Migrate Extras until it is in the contrib module, but it's not practical to remove it immediately (and more to the point, it's not necessarily simple for users of Migrate Extras to keep their module upgrades in lock-step). So we want a mechanism to say, for example, "use Pathauto's own migration handler but not Migrate Extra's".

Between the code registry and the handler's type registration, the necessary information exists, so this shouldn't be difficult (I've already started it).