Add --max-issues argument to limit requests

This addresses issue 71 by providing a configurable limit on the number of issues to retrieve. By default this limits the number of issues to 500 which should help users stay under the GitHub API limit and keep change logs from growing ridiculous in size for older repos.

lib/github_changelog_generator/parser.rb

@@ -24,6 +24,7 @@ module GitHubChangelogGenerator
           :enhancement_prefix => '**Implemented enhancements:**',
           :author => true,
           :filter_issues_by_milestone => true,
+          :max_issues => 500,
           :compare_link => true,
           :unreleased => true,
           :unreleased_label => 'Unreleased',
@@ -83,6 +84,9 @@ module GitHubChangelogGenerator
         opts.on('--exclude-labels  x,y,z', Array, 'Issues with that labels will be always excluded from changelog. Default is \'duplicate,question,invalid,wontfix\'') do |list|
           options[:exclude_labels] = list
         end
+        opts.on('--max-issues [NUMBER]', Integer, 'Max number of issues to fetch from GitHub. Default is 500') do |max|
+          options[:max_issues] = max
+        end
         opts.on('--github-site [URL]', 'The Enterprise Github site on which your project is hosted.') do |last|
           options[:github_site] = last
         end