Support Admonition Blocks

[BrooklinJazz]

Support the following syntax in Livebook to create an admonition block:

> #### Error {: .error}
>
> This syntax will render an error block

A few options for implementation:

1. Ask Livebook nicely to implement it on their side (or let us do it)
2. Write a mix task that finds these blocks in notebooks and replaces them with an HTML styled equivalent.