Does not support leading spaces

[Scott Main]

This works:

- one
- two

But this does not:

  - one
  - two

And it should.

According to the MD spec, it should tolerate up to three leading spaces:

List markers typically start at the left margin, but may be indented by up to three spaces.

Thanks for the great extension! I understand the sort of behavior provided by your extension require some exceptions to the rules, but it seems like this sort of behavior could still be allowed.