Added support for finding entry by terminologic code in find_by_code

There are scenarios where a terminologic code like ```deu``` is given and you want to convert this into a bibliographic one like ```ger```. Thatswhy it would be nice, if one could do something like

```ruby
ISO_639.find_by_code("deu")
=> ["ger", "deu", "de", "German", "allemand"]
```