Add support for regional FCE carrier data

The "fce" attribute has been removed from the carrier documents, and instead FCE data can be found as a YAML file inside datasets/:region/fce/:carrier.yml. This allows different regions to have their own FCE values.

```ruby
Carrier.find(:coal).fce(:nl) # => { australia: { ... }, ... }
Carrier.find(:coal).fce(:eu) # => nil
```