Extract EE specific files/lines for lib/banzai

We have the following files containing EE specific code. We should move them to ee/

• lib/banzai/pipeline/gfm_pipeline.rb
• lib/banzai/pipeline/post_process_pipeline.rb
• lib/banzai/pipeline/single_line_pipeline.rb

Those should be pretty straightforward, just need to make the filters list extensible.