‘--- ’ Dividing Line Support

[Jyong]

Is your feature request related to a problem? Please describe.
In markdown , we can use '* * * ' , '___' and '---' to generate dividing line . It seems that the commonmark only support transform "* * * " and ' ___ ' to ' hr ' label.

Describe the solution you'd like
Pls help to support '---'

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context about the feature request here.