Markdown bullet list command strips numbers from the start of lines

[b-kelly]

Reported on Meta.SE here.

Describe the bug

Using the unordered list command in markdown mode trims numbers from the start of the string.

To Reproduce

Add text to markdown mode like the following:

3 users need to foo the bar
then the bar becomes foo
4 bars have been foo'd

Highlight all 3 lines, then click the "bulleted list" command button.

The resulting text looks like:

- users need to foo the bar
- then the bar becomes foo
- bars have been foo'd

but should look like:

- 3 users need to foo the bar
- then the bar becomes foo
- 4 bars have been foo'd