Support extensions (documentation)

[moelders]

Hello, I have been facing the showdown extensions using this plugin, and I think it is a good idea to update the documentation of this plugin. You can see my changes.

Changelog:

• Update options object documentation with extensions.
• Update showdown to 1.9.0.
• Add extensions documentation in the guide section.
• Add package-lock.json.

Pending tasks:

• Translate the new extensions section in the docs/zh/guide/index.md.
• Build distribution files and documentation.

[meteorlxy]

@moelders Thanks for your PR! I'll check in detail later.

BTW, is it a good idea to have both package-lock.json and yarn.lock in a same repo?

[moelders]

As I do not use yarn I am not sure about the answer. If you prefer to rely on yarn I can remove the package-lock and add it to the .gitignore.

[moelders]

I have just reading this Yarn post about compatibility with package-lock from version 1.7.0.

But no problem, I can remove it and you can update yarn.lock.

[meteorlxy]

I didn't include extensions in the docs because I was lazy I thought extensions is also one of the property of options. But yeah it would be better to mention it in our docs, too.

I'll merge this PR after some tweaks and transilations. 😄

[meteorlxy]

@moelders

I've bumped vue-showdown to v2.2.0 with some breaking changes, which makes the usage of extensions different.

It will be nice if you have time to check them and give some suggestions. Thanks in advance 😄

[moelders]

Great @meteorlxy! I will check it in the next project update. Thanks!