Added :default param for I18n.localize

When using I18n.l, there are guards needed when a date is potentially nil:

```ruby
date && I18n.l(date)
date ? I18n.l(date) : 'present'
```

This change will return the default when passed in:

```ruby
I18n.l(date, default: nil)
I18n.l(date, default: 'present')
```