default -
similar to GFM, used when no preset name given. Enables all available rules,
but still without html, typographer & autolinker.
"zero" -
all rules disabled. Useful to quickly setup your config via .enable().
For example, when you need only bold and italic markup and nothing else.
MarkdownIt provides named presets as a convenience to quickly enable/disable active syntax rules and options for common use cases.
.enable(). For example, when you need onlyboldanditalicmarkup and nothing else.