Only include paragraph with indent of 4+ spaces or a tab in a list item

I reverted to the code in the original project [1] for the detection of
whether a paragraph separated from a list item by a space should be
included in a list item.

[1] "Joining only indented stuff after empty lines"
in parse_listitem
in markdown.c
in faelys/libsoldout
https://github.com/faelys/libsoldout/blob/e106f0d74a03fb5edbebe11ce4b46eef7f575456/markdown.c#L1145