Adds ability to create a MenuItemFont with Menu

[casper91]

Possible to create a MenuItem from a string.

Usage:

menu = Menu.new
menu.font_item('New Game') { puts 'new game' }
menu.font_item('High Score') { puts 'high score' }
menu.font_item('Exit Game') { puts 'exit game' }
menu.align_items_vertically(25)
add menu, 1