Allow adding custom classes to the admonition

[oprypin]

For example, to support this feature: https://squidfunk.github.io/mkdocs-material/reference/admonitions/#inline-blocks

I currently don't have any ideas regarding how it would still make sense with "graceful degradation", there's just no way...

This (made up) syntax makes little sense to me, and anything similar would probably be just as noisy:

NOTE {.inline .end}: Hello, world!

So perhaps this would be the syntax, relying on attr_list:

NOTE: Hello, world!
{.inline .end}

> NOTE: Hello, world!
>
> Foo bar
{: #someid .someclass somekey='some value' }

Currently the latter produces this HTML, which is no good:

<div class="admonition note">
  <p class="admonition-title">
    Note
  </p>
  <p>
    Hello, world!
  </p>
  <p class="someclass" id="someid" somekey="some value">
    Foo bar
  </p>
</div>

But in fact, the current state is consistent with how blockquotes behave! -- regardless of the callout part, the classes get attached to the last paragraph of the blockquote. So I have some doubts on trying to do it in this exact way.

But in any case, we would need to be able to produce this HTML:

<div class="admonition note someclass" id="someid" somekey="some value">
  <p class="admonition-title">
    Note
  </p>
  <p>
    Hello, world!
  </p>
  <p>
    Foo bar
  </p>
</div>