`middle-word-em` incorrectly triggers on `\`

[justanotheranonymoususer]

Version: 8c74601

import markdown2

# Expected output:
# <p><em>x</em>/<em>y</em> and <em>x</em>\<em>y</em></p>
# Actual output:
# <p><em>x</em>/<em>y</em> and <em>x*\*y</em></p>
markdown_text = R"""
*x*/*y* and *x*\\*y*
"""
print(markdown2.markdown(markdown_text, extras={'middle-word-em': {'allowed': False}}))

Also, it'd be nice for middle-word-em to only work on _, not *