RFC: change syntax of yfm-notes (move to directive syntax)

[d3m1d0v]

Description

The syntax of YFM-Notes is quite complex and inconvenient. There are several issues with it:

• curly liquid-like syntax for open and close block. It requires its exact spelling, empty lines before and after the content, named closing syntax
• strange behavior of title and autotitle
  • adds autotitle by default
  • "" – autotitle is disabled
  • "<title>" – adds custom title

Proposal

First, add support of directive syntax for YFM-Notes block: replace curly markup of opening and closing with directive syntax.

Example:

:::info
This is info.
:::

:::tip
This is a tip.
:::

:::warning
This is a warning.
:::

:::alert
alert
:::

Second, disable the auto-title by default and add "autotitle=true" attribute to enable it manually.