Add --base option

lib/github_changelog_generator/generator/generator_generation.rb

@@ -16,6 +16,8 @@ module GitHubChangelogGenerator
         log += generate_log_for_all_tags
       end
 
+      log += File.read(@options[:base]) if File.file?(@options[:base])
+
       log += "\n\n\\* *This Change Log was automatically generated by [github_changelog_generator](https://github.com/skywinder/Github-Changelog-Generator)*"
       @log = log
     end

lib/github_changelog_generator/parser.rb

@@ -50,6 +50,9 @@ module GitHubChangelogGenerator
         opts.on("-o", "--output [NAME]", "Output file. Default is CHANGELOG.md") do |last|
           options[:output] = last
         end
+        opts.on("-b", "--base [NAME]", "Optional base file to append generated changes to.") do |last|
+          options[:base] = last
+        end
         opts.on("--bugs-label [LABEL]", "Setup custom label for bug-fixes section. Default is \"**Fixed bugs:**""") do |v|
           options[:bug_prefix] = v
         end
@@ -153,6 +156,7 @@ module GitHubChangelogGenerator
         tag2: nil,
         date_format: "%Y-%m-%d",
         output: "CHANGELOG.md",
+        base: "HISTORY.md",
         issues: true,
         add_issues_wo_labels: true,
         add_pr_wo_labels: true,