Markdown bug: punctuation marks break italics formatted with underscores

In GitHub markdown, and in a previous version of Jupyter (don't remember which, sorry), it was possible to e.g. display quotations in italics formatted with underscores "_like this_". Unfortunately, Jupyter 1.0.0 displays this as "\_like this\_". The other formatting options work:

- "*asterisk for italics*"
- "__underscore for bold__"
- "**asterisk for bold**"

The error manifests not just with quotation marks, but with other punctuation marks (comma, dot, quastion mark, etc.) as well.

Reproduce the report in Jupyter 1.0.0 using underscores around italic text followed by quotation marks and other punctuation, then compare the asterisk cases described in the issue. Trace the notebook's Markdown rendering path; the work is done when punctuation no longer causes underscore-delimited italics to display as escaped text.

Written by the indexing model from the issue text.