[FR] ox-md: Export custom-id search strings (::#) as anchors

[Khalid Rafi]

=ox-md= currently can't export links like ~path::#custom-id~. But, I
think it's a very expected behavior to export such links as
~path#custom-id~ which is known as anchor links. I'm not sure why this
case was left alone.
-- 
Khalid Rafi
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