Introduce `disambiguate: true` option for formatting

This PR contains an extra option for formatting money. There are many currencies with the same symbols. When money is formatted in such a currency, the result is ambiguous. This problem arises with dollars (USD vs. CAD), pounds (GBP vs. FKP) and krona (SEK, DKK and NOK).

Usage:

```ruby
Money.new(100, "USD").format(:disambiguate => false)   #=> "$100.00"
Money.new(100, "CAD").format(:disambiguate => false)   #=> "$100.00"
Money.new(100, "USD").format(:disambiguate => true)    #=> "$100.00"
Money.new(100, "CAD").format(:disambiguate => true)    #=> "C$100.00"
```

The currency JSON files are extended with a `disambiguate_symbol` field. Each currency with an ambiguous symbol should have this field, containing an alternative symbol. When such a symbol is not present, the ISO code for the currency is used.

Thanks for reviewing and merging!