Fix nested list syntax

[bjcubsfan]

For markdown, Github indents bullets in a list so that it the items are nested if each level is indented two spaces like this:

Input text:

* Testing Testing, 1, 2, 3
  * Testing Testing, 1, 2, 3
    * Testing Testing, 1, 2, 3
      * Testing Testing, 1, 2, 3
        * Testing Testing, 1, 2, 3
          * Testing Testing, 1, 2, 3

Rendered:

• Testing Testing, 1, 2, 3
  • Testing Testing, 1, 2, 3
    • Testing Testing, 1, 2, 3
      • Testing Testing, 1, 2, 3
        • Testing Testing, 1, 2, 3
          • Testing Testing, 1, 2, 3

I didn't see this in the markdown spec, but I would like markdown-preview to do this. Or maybe there should be a version for Github flavored markdown.

Thanks!

[fabb]

+1