Allow optimizer to be pluggable

[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, compliance_level; 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.