FEATURE: adds countdown attribute to [date] (#8037)

When used, dates will be displayed as a countdown in a human friendly way.

plugins/discourse-local-dates/spec/integration/local_dates_spec.rb

@@ -79,4 +79,11 @@ RSpec.describe "Local Dates" do
 
     expect(cooked).not_to include("data-timezone=")
   end
+
+  it 'supports countdowns' do
+    raw = "[date=2018-11-01 time=12:00 countdown=true]"
+    cooked = Fabricate(:post, raw: raw).cooked
+
+    expect(cooked).to include("data-countdown=")
+  end
 end