added the possibility to use a proc or lambda as index_name

This was the only thing missing to make searchkick compatible with my multi-tenanted app.
Using [Apartment](https://github.com/influitive/apartment), I can just do this : 
```ruby
searchkick index_name: -> { "#{self.name.tableize}-#{Apartment::Database.current_tenant}" }
```

...and voilà!