v3.0.0-preview4

This is probably the last v3 preview release before it's released as stable.

Changelog

• Refactor how variables work on version 3 (#311).
• Disallow expansions on v3 since it has no effect.
• Taskvars.yml is not automatically included anymore.
• Taskfile_{{OS}}.yml is not automatically included anymore.
• Allow interpolation on includes, so you can manually include a Taskfile based on operation system, for example.