Allow redirects from and for subclasses of page and document

[radoslavkarlik]

Implements the issue #203

[parkr]

Thank you so much! The code looks great. Let's get a quick test in here.

In spec/jekyll_redirect_from/generator_spec.rb, let's add:

context "redirectable_document?" do
  let(:generator) { JekyllRedirectFrom::Generator.new }
  it "accepts subclasses of Jekyll::Document" do
    SubclassOfJekyllDocument = Class.new(Jekyll::Document)
    expect(generator.send(:redirectable_document?, SubclassOfJekyllDocument.new)).to be_truthy
  end

  it "accepts subclasses of Jekyll::Document" do
    SubclassOfJekyllPage = Class.new(Jekyll::Page)
    expect(generator.send(:redirectable_document?, SubclassOfJekyllPage.new)).to be_truthy
  end
end

I haven't run the above code but hopefully it gets you started. 🚀

[radoslavkarlik]

Sorry guys, busy life happened. I have added the tests with a little modification.

[DirtyF]

@jekyllbot: merge +minor