Add experimental support for dynamic values

This allows a submission to get a dynamic value like for example the current time

app/models/submission.rb

@@ -6,11 +6,19 @@ class Submission < ApplicationRecord
   def data=(value)
     sanitized_data = {}
     value.each do |key, value|
-      sanitized_data[key] = value.to_s
+      sanitized_data[key] = submission_value(value)
     end
     write_attribute(:data, sanitized_data)
   end
 
+  def submission_value(value)
+    if value.to_s.downcase == 'tinyforms_now'
+      Time.now.utc.to_formatted_s(:rfc822)
+    else
+      value.to_s
+    end
+  end
+
   def append_to_spreadsheet
     result = form.append(data)
     update_column(:appended_at, Time.current) if result.updates.updated_rows > 0