[lib][Markdown] Terminate lists with two newlines
[lib][Markdown] Terminate lists with two newlines

Summary:
I think terminating the list after twolines lines up a little better with most people's expectations.

With this change, it's still possible to have a list item containing a newline, you just need to add a couple of spaces after the newline.

@KatPo, would appreciate any help testing this and making sure it doesn't break things.

Reviewers: KatPo, palys-swm

Reviewed By: KatPo, palys-swm

Subscribers: zrebcu411, Adrian, KatPo

Differential Revision: https://phabricator.ashoat.com/D67

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal