Allow to configure custom block levels

[oscarotero]

Hi! In my projects, I'm using markdown-it-container allowing to create block-level custom containers within markdown. The markup syntax is the same as for fenced code blocks. Difference is, that marker use another character and content is rendered as markdown markup. As an example:

This is a paragraph

:::block
This is a custom container block
:::

This is another paragraph.

I'd love to have a way to configure these new blocks, defining the character used. For example:

container_chars: [':']

Thank you for this great project.

[jonschlinkert]

This is an interesting idea, I'd be open to it if someone else wanted to do a pr for it, and there were some kind of tests implemented (fwiw this is what holds up virtually every decision I make on this lib. I'm not familiar with ST enough to have confidence in code changes without tests).