When using one of the `*_list` methods, allow the block to receive array
index for each object:
```ruby
posts = build_list(:post, 10, title: 'Post number') do |post, i|
post.title = "#{post.title} #{i + 1}"
end
posts.first.title # => "Post number 1"
posts.last.title # => "Post number 10"
```
A block that only takes in the object still works as before, as does
leaving off the block entirely.
Co-authored-by: Mike Countis <mike.countis@gmail.com>
Emmanuel Gautier