Configurable min length

[TheoMcCabe]

trafficstars

I think we should be able to configure the min length of container markers

by default if we set the mark to :

::: container 
:::

is a container but

:: container 
::

is not.

Some popular libraries like nuxt content use only 2 marks to signify a container, so it would be nice to be able to configure this

Im happy to submit a PR for this