Make pipelines actually make sense

lib/gitlab/markdown.rb

lib/gitlab/markdown/asciidoc_pipeline.rb

lib/gitlab/markdown/atom_pipeline.rb

lib/gitlab/markdown/combined_pipeline.rb

lib/gitlab/markdown/description_pipeline.rb

lib/gitlab/markdown/email_pipeline.rb

lib/gitlab/markdown/full_pipeline.rb

lib/gitlab/markdown/gfm_pipeline.rb

lib/gitlab/markdown/note_pipeline.rb

lib/gitlab/markdown/pipeline.rb

lib/gitlab/markdown/plain_markdown_pipeline.rb

lib/gitlab/markdown/post_process_pipeline.rb

lib/gitlab/markdown/reference_extraction_pipeline.rb

lib/gitlab/markdown/single_line_pipeline.rb