Improve numbered lists

[sam (Sam Saffron)]

99. bugs of beer on the wall

Nintey nine bugs of beer, eat one bug, what do you have?

98. bugs of on wall

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99. bugs of beer on the wall 

Nintey nine bugs of beer, eat one bug, what do you have? 

98. bugs of :beers: on wall

CommonMark testing started here!

Per our migration plans, I just enabled markdown.it at Meta. It is an excellent CommonMark compliant engine. I expect quite a few edges to be found during this period of testing. Do not open bugs about the new Engine, just reply here. I/we will be updating the top post here with the list of issues and striking out stuff as it is fixed. Current known issues (to be fixed) Onebox vanishes after you start typing and works not-super consistently pendin…