Add `Money.default_formatting_rules` config hash

I think having a way to define default rules for everytime `Money#format` is called is a pretty useful feature.

This is basically how it works:
```ruby
Money.default_formatting_rules = { display_free: true }

Money.new(0, 'USD').format
# => "free"

# Default rules can be overwritten when provided on method call:
Money.new(0, 'USD').format(display_free: false)
# => "$0.00"
```