MD007: Check for four spaces for nested lists

[koppor]

When mixing lists and enumarations, the sub list should have an indent of 3 spaces:

1. example1

   - sub 11
   - sub 12

2. example2

   - sub 21
   - sub 22

In case one indents with four spaces, there is no error reported:

1. example1

    - sub 11
    - sub 12

2. example

    - sub 21
    - sub 22

There should be a warning, that three spaces are enough.

Howver, two spaces are too less since that does not work, "because the nesting of two spaces is insufficient to push it into the ordered list item"). This is catched by MD007 and thus shouldn't be an issue.

(This is a follow-up to https://github.com/DavidAnson/markdownlint/issues/20#issuecomment-621427480)