Add --since-tag option

Fix #254

lib/github_changelog_generator/generator/generator_tags.rb

@@ -56,10 +56,29 @@ module GitHubChangelogGenerator
     #
     # @return [Array]
     def get_filtered_tags(all_tags)
-      filtered_tags = filter_between_tags(all_tags)
+      filtered_tags = filter_since_tag(all_tags)
+      filtered_tags = filter_between_tags(filtered_tags)
       filter_excluded_tags(filtered_tags)
     end
 
+    def filter_since_tag(all_tags)
+      filtered_tags = all_tags
+      tag = @options[:since_tag]
+      if tag
+        if all_tags.map(&:name).include? tag
+          idx = all_tags.index { |t| t.name == tag }
+          if idx > 0
+            filtered_tags = all_tags[0..idx - 1]
+          else
+            filtered_tags = []
+          end
+        else
+          Helper.log.warn "Warning: can't find tag #{tag}, specified with --since-tag option."
+        end
+      end
+      filtered_tags
+    end
+
     def filter_between_tags(all_tags)
       filtered_tags = all_tags
       if @options[:between_tags]