Escape unicode newlines when compiling

[TheDancingCode]

Summary

Proposed change:

Escapes the \u2028 unicode newline character to avoid errors when compiling.

Closes #126, closes #736 .

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[codecov-io]

Codecov Report

Merging #1126 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1126      +/-   ##
==========================================
+ Coverage   89.93%   89.94%   +<.01%     
==========================================
  Files          22       22              
  Lines        2982     2983       +1     
==========================================
+ Hits         2682     2683       +1     
  Misses        300      300

Impacted Files	Coverage Δ
nunjucks/src/compiler.js	95.51% <100%> (ø)	⬆️

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[fdintino]

Thanks for this! Apologies for the delay in merging, it escaped my notice back in July.

[fengmk2]

unicode newlines worked on node >= 10