Bullets do not continue on newlines in MarkdownEditor

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When editing a bulleted list, hitting return should continue the list. Hitting return again should remove the new bullet.

When I wrote MarkdownTwain originally, I noticed this issue but opted not to fix it right away due to complexity. We want bulleted lists to continue if you press return, but not if you paste a bulleted list from a Markdown document in your paste buffer or paste history. This makes the necessary solution more elaborate then simply continuing the list on newlines.