Taskwarrior integration node

[ForNeVeR]

There should be a fully-fledged integration with Taskwarrior:

• we should be able to read all the changes from Taskwarrior instance
• we should be able to export changes from Praefectus back to the Taskwarrior
• merge changes on import/export properly
• search for data conflicts of any kind, and report them to the user

Depends on:

• Configuration system #10 (since we need configuration to seek for Taskwarrior instance and to know whether the integration is enabled at all)
• Simple data storage #9 (since we need to store data a lot)
• Foundational concept design: task attributes #6 (since we need to know what to store)
• Foundational concept design: multi-node support #8 (since Taskwarrior integration is a separate node by design)
• Foundational concept design: version control #7 (since we need to know what/how to merge)