Allow for `:::admonition` syntax (without curly brackets)

Right now our special admonition syntax uses the same curly-bracket structure that we use for directives. AKA:

```{note}
my note
```

is equivalent to

:::{note}
my note
:::

What do people think about supporting the Pandoc-style of admonition classes w/o curly brackets:

:::note
my note
:::

This could be a comma-separated list of admonition labels, so we could allow for :::note,admonition and this would behave like :::{note,admonition}.

One benefit of this would potentially be interoperability with other markdown specs. For example, in HackMD :::info blocks will render as special “info” blocks, whereas the curly brackets are a more MyST-specific thing so less likely to interoperate with other flavors of markdown.

[chrisjsewell]

note also parsing direct url links (like https://example.com) is coming soon: https://github.com/executablebooks/markdown-it-py/pull/78

[chrisjsewell]

I’m closing this, because in #273 ::: now act exactly the same as ```, which precludes not having the curly braces