Add "forceescape" filter

[TheDancingCode]

Summary

Proposed change:

Adds the forceescape filter.

Closes #782.

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 #1090 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1090      +/-   ##
==========================================
+ Coverage   89.92%   89.93%   +0.01%     
==========================================
  Files          22       22              
  Lines        2979     2982       +3     
==========================================
+ Hits         2679     2682       +3     
  Misses        300      300

Impacted Files	Coverage Δ
nunjucks/src/filters.js	96.11% <100%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update de49d33...f478b06. Read the comment docs.

[fdintino]

Looks great. Thanks for your contribution!