Is there any way I can change default sort key order 'desc' to 'asc'?

Hello, thanks for great library :)

I'm facing a problem of forcing `active_admin` to generate links inside 'th' tags with default `asc` key instead of `desc`.
I have followed source code till  [this point](https://github.com/gregbell/active_admin/blob/a25c9231715a4124bd7a303f29feeb381ec88956/lib/active_admin/views/components/table_for.rb#L136) but it gave no tips how can I override default logic.

```ruby
# models/post.rb
# Post
# t.text :body
# create_table :posts
# t.string :title

# admin/post.rb
ActiveAdmin.register Post do
  index do
    column :title
    column :body
    actions
  end
end
```

My goal is to see this type of link generated `/admin/posts?order=title_asc` instead of default `/admin/posts?order=title_desc`. So when user clicks on column he wil receive `asc` results.