Add NodeResolveLoader

[fdintino]

Summary

Adds NodeResolveLoader, a Loader that loads templates using node's require.resolve.

Fixes #1175

Checklist

• 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).

[fdintino]

The appveyor failure seems to indicate that I'm not using cross-env correctly to test this on windows. I'll need to investigate more later.

[codecov-io]

Codecov Report

Merging #1197 into master will decrease coverage by 0.21%.
The diff coverage is 64%.

@@            Coverage Diff            @@
##           master   #1197      +/-   ##
=========================================
- Coverage   89.81%   89.6%   -0.22%     
=========================================
  Files          22      22              
  Lines        2995    3020      +25     
=========================================
+ Hits         2690    2706      +16     
- Misses        305     314       +9

Impacted Files	Coverage Δ
nunjucks/index.js	64.7% <ø> (ø)	⬆️
nunjucks/src/node-loaders.js	68.65% <64%> (-2.78%)	⬇️

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[codecov-io]

Codecov Report

Merging #1197 into master will decrease coverage by 0.21%.
The diff coverage is 64%.

@@            Coverage Diff            @@
##           master   #1197      +/-   ##
=========================================
- Coverage   89.81%   89.6%   -0.22%     
=========================================
  Files          22      22              
  Lines        2995    3020      +25     
=========================================
+ Hits         2690    2706      +16     
- Misses        305     314       +9

Impacted Files	Coverage Δ
nunjucks/index.js	64.7% <ø> (ø)	⬆️
nunjucks/src/node-loaders.js	68.65% <64%> (-2.78%)	⬇️

---

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 c99154e...c3741c9. Read the comment docs.