Refactor application data, introduce migration path.
Add head command.
Fix the specs.

* Stub XDG[HOME] instead of LegacyAppData.repofile.
* Fix the default value returning in LegacyAppData.
Add specs for head.
Merge branch 'feature/head-command' into develop
Specs: Initialize AppData instead of creating the yaml file manually.
Bump version to 0.3.0
Merge branch 'release/0.3.0' into develop