Enable additional markdown plugins#26

The mistune library contains a few very useful plugins that are already included in the package: https://mistune.lepture.com/en/latest/plugins.html#built-in-plugins

e.g. task-lists or

I’ve gone and enabled the following plugins (and will update the docs to reflect this):

• strikethrough (~~here is the content~~)
• footnotes (`[^1]: footnote explain)
• table
• auto url (when using <https://example.com> format)

The other plugins interfere with content a bit too much (like mark will break anyone’s content who has writen some example js cause of the ==)

It would be great to add the ability to add classes within Markdown, like in Kramdown.

So this:

Some text here {: .notice}

Would render as:

<p class="notice">Some text here</p>

I’ll consider kramdown syntax, but in the meantime you can just write that in pure HTML since Bear supports mixing MD and HTML