Configurable min length

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 