Add `LocoMenu#print_items`
Remove `menu_helper`

Add `items` instance variable.
Make `print_items` handle all about printing items
Add `SelectBoardMenu#print_boards`

Add `cur_boards` instance variable.
Simplify some expression
Use guard clause with `term.beep`
Refresh only changed line in `SelectBoardMenu`
Add `process_key` method for key input

- LocoMenu
- SelectBoardMenu
fix bugs in unicode_slice
Merge pull request #59 from yous/patch-cyclomatic-complexity

Reduce cyclomatic complexity
Optimize `#unicode_slice`