Official support for being able to pass multiple key-value pairs to the find method

[octopusinvitro]

Based on this discussion, regarding officially supporting the possibility to pass multiple key-value pairs to the find method of countries and legislatures, do we want to offer the possibility of:

1. searching for a country or legislature by more than one field at the same time?, for example:

country = Everypolitician::Country.find(country: 'Abkhazia', code: 'GE-AB')

2. or would we only support searching by several fields but just one at a time? For example:

country = Everypolitician::Country.find(name: 'Australia')

3. or just by slug?

country = Everypolitician::Country.find('Australia') # Default behavior ATM

In cases 2. and 3., it would be nice to simplify the code to the behaviour we want. For example, if we only support 3., one possible simplification (for legislatures) could be:

    def legislature(query)
      query = { slug: query } if query.is_a?(String)
      legislature = legislatures.find do |l|
        query.all? { |k, v| l.__send__(k).to_s.downcase == v.to_s.downcase }
      end
      fail Error, "Unknown legislature: #{query}" if legislature.nil?
      legislature
    end

with:

    def legislature(query)
      legislature = legislatures.find do |l|
        l.__send__(:slug).to_s.downcase == query.to_s.downcase }
      end
      fail Error, "Unknown legislature: #{query}" if legislature.nil?
      legislature
    end

This would also be more in line with how we documented the code in the README, where we are telling people to pass the slug to the method.