Allow option to preserve the sort order as inputted in the only array of countries

[pmor]

@scudco

I've been thinking about #194 , and came up with another possibility:

country_select :user, :country_code, only: %w(CH AT DE), sort_provided: false

country_select :user, :country_code, priority_countries: %w(CH AT DE), sort_provided: false

That is, adding a sort_provided option that controls if the list of countries is locale-sorted or we use the order of the only and priority_countries array.

The default behaviour is unchanged from the current version, this just adds the new option to allow preserving the sort order as inputted, and is a pretty small code change.

This adds the new option and is a small change, but it changes the default behaviour for priority_countries to sorting the list, so that both priority_countries and only have the same sorting behaviour and defaults, so it does include a breaking change.

[pmor]

For clarity, the swapping the order of the arrays here is what preserves the order of the only array. The new option is there to control if the final output is sorted by country_options_for or not