Bugfix/fix string newlines

[joseph-roitman]

Fixes support for string snapshots containing newlines.

Fixes #31 .

[codecov]

Codecov Report

Merging #32 (7e41ec4) into master (9b85511) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   99.37%   99.38%           
=======================================
  Files           7        7           
  Lines         321      326    +5     
  Branches       35       35           
=======================================
+ Hits          319      324    +5     
  Misses          2        2           

Impacted Files	Coverage Δ
pytest_snapshot/plugin.py	100.00% <100.00%> (ø)
tests/test_assert_match.py	100.00% <100.00%> (ø)

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