Changes:
- Rename the UI type to App.
- Rename NewUI() to NewApp().
- Move init functions to internal/ui/ui.go.
- Call app's initialisation function from main.
- Move the keymapping functionalities to internal/ui/keymappings.go.
- Add documentation for updateColumn() and updateAllColumns().
- Applied gofmt.
Also:
- Build(mage): optionally include -a and/or -v during go build.
