Add: %u format flag to show ?? if total is unknown

lib/ruby-progressbar/format/molecule.rb

@@ -6,6 +6,7 @@ class   Molecule
     :T => [:title_comp,   :title],
     :c => [:progressable, :progress],
     :C => [:progressable, :total],
+    :u => [:progressable, :total_with_unknown_indicator],
     :p => [:percentage,   :percentage],
     :P => [:percentage,   :percentage_with_precision],
     :j => [:percentage,   :justified_percentage],

lib/ruby-progressbar/progress.rb

@@ -100,6 +100,10 @@ def unknown?
     progress.nil? || total.nil?
   end
 
+  def total_with_unknown_indicator
+    total || '??'
+  end
+
   def percentage_completed_with_precision
     return 100.0  if total == 0
     return 0.0    if total.nil?

spec/ruby-progressbar/base_spec.rb

@@ -800,6 +800,18 @@
         expect(progressbar.to_s('%C')).to match(/^100\z/)
       end
 
+      it 'displays nothing when the total is unknown and when passed the "%C" format flag' do
+        progressbar = ProgressBar::Base.new(:total => nil)
+
+        expect(progressbar.to_s('%C')).to match(/^\z/)
+      end
+
+      it 'displays ?? when the total is unknown and when passed the "%u" format flag' do
+        progressbar = ProgressBar::Base.new(:total => nil)
+
+        expect(progressbar.to_s('%u')).to match(/^\?\?\z/)
+      end
+
       it 'displays the percentage complete when passed the "%p" format flag' do
         progressbar = ProgressBar::Base.new(:starting_at => 33, :total => 200)