feat(toc_obj): Support unnumbered headings

[Cerallin]

This PR adds a feature for unnumbered headings.

1. The user specifies unnumbered_class_list and passes it to tocObj.
2. Headings WITHOUT classes in unnumbered_class_list will be parsed as usual.
3. Headings WITH classes in unnumbered_class_list will be parsed with unnumbered: true.

As is provided in test/toc_obj.spec.js, I have made minimized modifications as much as possible.

P.S. Maybe unnumbered_class_list is too long?

[yoshinorin]

@Cerallin
Would you please update README?

https://github.com/hexojs/hexo-util/tree/ab5ad9dd906f865faeba846b92be9dcce8c27120#tocobjstr-options

P.S: no need force-push. we will squash-merge.

[yoshinorin]

@Cerallin
Would you please fix lint?

[yoshinorin]

@Cerallin
Continues from hexojs/hexo#4871 (comment)

Do you have any idea to use this option with other renderers? Currently, hexo-renderer-marked and hexo-renderer-markdown-it don't have an option to add custom attributes. They are the official renderer of hexo. IMHO, it is better to use this option with official renderers.

If difficult to implement it, would you please update the docs that users need to customize (extends) the renderer by themselves to add an attributes.

I assume users will be confused "this option can be used in official renderers by default".

@SukkaW
Do you have any suggestions?

[Cerallin]

@yoshinorin
This is a plugin for markdown-it: markdown-it-attrs.
And for marked, maybe we need to implement the feature by ourselves.

[Cerallin]

Support for markdown-it-attrs is added via hexojs/hexo-renderer-markdown-it#174.