Feature request: support default processors in configuration

[ianyamey]

There's currently no way to configure default HTML processors globally. If you want a processor to run on every markdownify call, you have to either:

1. Pass process: [MyProcessor] to every markdownify call in every view
2. Prepend Perron::MarkdownHelper to inject defaults — which works, but feels like it should be a first-class config option

For example, I have a processor that fixes an accessibility issue with CommonMarker's anchor links (adding tabindex="-1" to aria-hidden="true" anchors). This needs to run on all rendered markdown, so I'm currently doing:

Rails.application.config.to_prepare do
  Perron::MarkdownHelper.prepend(Module.new do
    def markdownify(content = nil, process: [], &block)
      super(content, process: process + [AnchorTabindexProcessor], &block)
    end
  end)
end

Proposal

Add a default_processors configuration option:

Perron.configure do |config|
  config.default_processors = [AnchorTabindexProcessor, "target_blank"]
end

These would be prepended to whatever is passed via process: in individual markdownify calls, so per-call processors still work as expected:

<%# Runs AnchorTabindexProcessor, target_blank (from defaults), plus lazy_load_images %>
<%= markdownify @resource.content, process: %w[lazy_load_images] %>

Implementation

This could be as simple as:

1. Adding default_processors to Perron::Configuration (defaulting to [])
2. Merging them in MarkdownHelper#markdownify:

def markdownify(content = nil, process: [], &block)
  text = block_given? ? capture(&block).strip_heredoc : content
  all_processors = Perron.configuration.default_processors + Array(process)
  Perron::Markdown.render(text, processors: all_processors)
end

[eelcoj]

I have been thinking about this as well. I have the same need and have been copying the array with processors around.

The only thing I am uncertain about is prepending with the explicit added processors. Is it expected? Can it cause issues? Is it flexible enough?

I am leaning towards explicitly defined in markdownify overrides the default ones. Maybe this is just semantics about the name default_processors.

But I like to see something like this added. ✨