Allow optimizer to be specified using the :pdf_optimizer option

[mojavelinux]

Currently, it's only possible to override the optimizer by implementing (or patching) the built-in Asciidoctor::PDF::Optimizer class (see README). Instead, it should be possible to pass a custom class or object to the API using the :pdf_optimizer option.

The interface for the Optimizer is:

class Optimizer
  def initialize quality, compatibility_level, compliance; end
  def optimize_file path; end
end

An alternative would be to allow the class to self register, similar to how the syntax highlighter adapters or converter work in core.

[mojavelinux]

I'm going to create a separate issue for allowing new optimizers to be registered by name, then activated using the pdf-optimizer attribute. This issue will be repurposed to add the :pdf_optimizer option to the API.

[mojavelinux]

The :pdf_optimizer option expects either a class or an object that responds to the new method.