Add flat tabs

Adding the ability to customize tab icons further with selected and unselected states.

Again, I didn't change the API at all. You can now pass an icon in as a hash with `:selected` and `:unselected` states.

For example:

```ruby
class BuddyScreen < PM::Screen
  def on_init
    icon = font_awesome_tab_icon(:group)
    set_tab_bar_item icon: {selected: icon, unselected: icon}, title: "Buddies"
  end
end
```

All previous icon types still work (strings for file names and UIImages for the default behavior).