Added option to extend existing arrays

Hi !

I added an option to extend existing arrays. Example

```ruby
source = { "property" => "4" }
dest   = { "property" => ["1", "2", "3"] }
dest.deep_merge!(source, {:extend_existing_arrays => true})
Results: {"property" => ["1", "2", "3", "4"]}
```

There is a test and I added docs to the README. Hope you find this useful. Personally, I need this very often.

Cheers