Make the plugin monkey-patching friendly

[Nowaker]

This code isn't friendly for monkey-patching:

Jekyll::Hooks.register :site, :pre_render do |site, payload|
  # non
  # monkeypatch-friendly
  # code
  # here...
  # :-(
end

This one is:

def pre_render site, payload
  # monkey-patching friendly code here, yay!
end

Jekyll::Hooks.register :site, :pre_render do |site, payload|
  pre_render
end

This is just one example of how this cane be achieved. A top-level def method is one option, a def self class method, and probably more.

Thanks!