GH editor-like (un)ordered list bullet auto insertion

Is your feature request related to a problem? Please describe.
On the Markdown mode, I expect:

1. Hitting enter seems like it would give me another number when I already am on a numbered line.
2. Hitting enter again I'd assume it would delete the number if no other text was present.
3. Hitting again (without the number on a line) I'd assume it would create a new line, as expected, normal enter behavior.

THIS Github EDITOR I'M IN RIGHT NOW DOES EXACTLY WHAT I'M ASKING FOR :)

We can do one better though, if we're on a line below a numbered line, say we're editing an existing list:
I expect that we would interpret the previous line and make the next number if there are no newlines or lines of text in between.
I don't think that would delay things too much since you only have to look at the beginning of the line and the next couple of characters with a compiled regex or w/e.

No file or test is named. Start by locating the Markdown mode input and list-handling logic, then trace how Enter currently processes numbered and unordered list lines. Done means matching the described continuation, list-exit, and normal-newline behavior, with coverage for each case.