Add option to enable toc by default

[gandalf3]

Perhaps it's only me, but I wanted a way to globally enable tables of contents and merely override it on a page-by-page basis. This PR adds an option for that, enable_by_default.

This PR also makes toc_only return nothing when toc is disabled. This might be
a more controversial change (which perhaps should be in a different PR), but I can't really think of a case where the previous behavior would be desired. I also realize this probably breaks some tests. Unfortunately, when I went to look, a whole bunch of tests seemed to be failing even on master. I recognize that it's a bit poor form to submit a PR without updating the tests, but I'm not too sure what's going on there (plus I'm not familiar with ruby at all).

[toshimaru]

You can configure default value on your _config.yml

defaults:
  -
    scope:
      path: "" # an empty string here means all files in the project
    values:
      layout: "default"
      toc: true # This means toc: true by default

ref. https://jekyllrb.com/docs/configuration/front-matter-defaults/

---

So, I'd say we don't need to implement it in jekyll-toc itself, please use your own _config.yml.

Thanks for sending PR!

[gandalf3]

Ah, I can't believe I didn't realize that. Thanks :P

It's a minor matter, but what is your opinion on making toc_only return nothing when toc is disabled? If you like, I can submit a new PR with just that change (though it's such a small change it hardly seems necessary).

[toshimaru]

what is your opinion on making toc_only return nothing when toc is disabled?

This is a good idea. 👍 (I'd say this behavior is a bug 😩 )

I can submit a new PR

Go ahead. :)

I think just a one-line change?

jekyll-toc/lib/jekyll-toc.rb

Line 19 in 6d4a27d

return html unless toc_enabled?

[gandalf3]

Yep, though again I'm not sure about tests. #77