Add country code filter options, closes #6

Added the command line flag `--country` (or `-c`) which allows you to filter the server list down to only servers in the specified country.
Refactor code to allow custom serverList

Before the code for loading and filtering the serverList was in the `query` function, this meant you couldn't override the server list in a simple way. Now there is a `prepareList` function that loads and filters the query list seperately to `query`.

The advantage of this is you can provide a custom serverList just by setting `dnsyo.lookup.serverList` on a class instance.

Woo flexibility.