Use correct marker from previous list level

Use correct marker from previous list level

Putting the point after "AAA" in the following sample and pressing `C-u`
`M-RET` should insert "*" (properly dedented) instead of "1.".

```
1. A
    * AA
        1. AAA
```

It should produce:

```
1. A
    * AA
        1. AAA
    *
```

Previously, it produced this instead:

```
1. A
    * AA
        1. AAA
    1.
```

This commit changes the behavior of `(markdown-prev-list-item level)`.
It now stops at list items with (non-list-marker) indentation at or
*below* `level`. In the example above, the (non-list-marker) indentation
of "AAA" is 11. Subtracting 4 gives 7, but the corresponding indentation
of the previous item "AA" is only 6. So, when mixing list types it does
not suffice to simply subtract 4 and look for equal indentation.
Instead, we subtract 1 and find the previous item with *lesser than or
equal* indentation.

Closes https://github.com/jrblevin/markdown-mode/issues/4