Changing only a portion of the settings

Hi,  

I needed a way to change the autocomplete nGram without loosing all the analyzers.  
Maybe changing from `merge!` to `deep_merge!` would help somebody else who also wants to change only a portion of the settings ;-)

Now I can do something like this in my models: 
    
    searchkick autocomplete: ['name', 'title'], settings: {
        analysis: {
            tokenizer: { 
                searchkick_autocomplete_ngram: { type: "nGram" }
            }
        }
    }