Added various endpoints for subscriptions

lib/convertkit/client/forms.rb

@@ -4,6 +4,10 @@ module Forms
       def forms
         connection.get("forms").body["forms"]
       end
+
+      def form_subscriptions(form_id, options = {})
+        connection.get("forms/#{form_id}/subscriptions", options)
+      end
 
       def add_subscriber_to_form(form_id, email, options = {})
         connection.post("forms/#{form_id}/subscribe") do |f|

lib/convertkit/client/sequences.rb

@@ -5,6 +5,10 @@ def sequences
         connection.get("sequences")
       end
 
+      def sequence_subscriptions(sequence_id, options = {})
+        connection.get("sequences/#{sequence_id}/subscriptions", options)
+      end      
+
       def add_subscriber_to_sequence(sequence_id, email, options = {})
         connection.post("sequences/#{sequence_id}/subscribe") do |f|
           f.params['email'] = email

lib/convertkit/client/tags.rb

@@ -5,6 +5,10 @@ def tags
         connection.get("tags")
       end
 
+      def tag_subscriptions(tag_id, options = {})
+        connection.get("tags/#{tag_id}/subscriptions", options)
+      end      
+
       def add_subscriber_to_tag(tag_id, email, options = {})
         connection.post("tags/#{tag_id}/subscribe") do |f|
           f.params['email'] = email