Add new danger_local Rake task

The benefit of running the danger_local Rake task
is that it displays some informational messages
for clarity and also exits with 1 if there's any
output.

lib/tasks/gitlab_danger.rake

+desc 'Run local Danger rules'
+task :danger_local do
+  require 'gitlab_danger'
+  require_relative '../../lib/gitlab/popen'
+
+  puts("#{GitlabDanger.local_warning_message}\n")
+
+  # _status will _always_ be 0, regardless of failure or success :(
+  output, _status = Gitlab::Popen.popen(%w{danger dry_run})
+
+  if output.empty?
+    puts(GitlabDanger.success_message)
+  else
+    puts(output)
+    exit(1)
+  end
+end