Allow exclusion of navigation links via the `paginate` method and globally with `config`

Allows the exclusion of certain nav links without having to generate and modify the view templates.

```ruby
# config/initializers/kaminari_config.rb
Kaminari.configure do |config|
  config.exclude [:first, :last]
end
```

```ruby
# app/views/users/index.html.haml
# ...
= paginate @users, :exclude => [:first, :last]
# ...
```

Supports `haml`, `erb`, and `slim` templates.

Closes #235, long dead but never implemented.