Add support for rendering admonitions using "markdown friendly" syntax

i.e. implement an extension in markdown-it-py to encapsulate something like pandoc fenced divs

:::{note}
my note
:::

or suggested in jupyter https://github.com/jupyter/notebook/issues/1292#issuecomment-477152896

!!! note
    This is a note.

Then render it here

[choldgraf]

Another quick thought here as I was playing around with writing this kind of syntax. If we support :::, then we probably don’t need the {curly-brackets} in that case. E.g. treat:

```{this}
```

equivalent to

:::this
:::

since we are not over-riding any other behavior with it.

I think other than that difference, the two patterns should behave exactly the same way. E.g. you could do

```{admonition} My title
:key: val
Content
```

and

:::admonition My title
:key: val
Content
:::

The one downside I can see there is that there isn’t an obvious separation between the directive name and its arguments

[chrisjsewell]

Yep the same way that #132 and #153 will be optionally activated, by making this more configurable: https://github.com/executablebooks/MyST-Parser/blob/8dda2f73364853cb572a50c53434c385db266d7c/myst_parser/main.py#L31-L45