Test backslash in markdown

[mwouts]

Test the LaTeX equation provided at #305 and make sure that the round trip works.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ec25f7e). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #363   +/-   ##
=========================================
  Coverage          ?   99.06%           
=========================================
  Files             ?       73           
  Lines             ?     7254           
  Branches          ?        0           
=========================================
  Hits              ?     7186           
  Misses            ?       68           
  Partials          ?        0

Impacted Files	Coverage Δ
tests/test_mirror.py	100% <ø> (ø)
tests/test_cli.py	100% <100%> (ø)
jupytext/cell_to_text.py	97.4% <100%> (ø)
tests/utils.py	100% <100%> (ø)
tests/test_contentsmanager.py	100% <100%> (ø)

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