[markdown] Specify type of list when highlightFormatting is enabled

Closes #2039. There was some confusion when the issue was opened, but the request was narrowed down [in the comments](https://github.com/marijnh/CodeMirror/issues/2039#issuecomment-30163517). It was originally [requested in another issue](https://github.com/marijnh/CodeMirror/issues/1919#issuecomment-29655125).

@davedawson and @lukasoppermann, want to take a look at this and verify all of the features are in now?