Consider associating VarIds with tasks

[cassaundra]

Currently, tasks are registered in a global scope managed by quake. By assigning variables to tasks, perhaps storing metadata inside of a custom value, we can get scope resolution and shadowing of tasks for free.

This would also allow for referencing tasks with other commands or as values (e.g. $build.artifacts). This could alternatively be implemented with the existing global $quake variable (e.g. $quake.tasks.build.artifacts).