Add reject filter

[ogonkov]

Summary

Proposed change:

Add reject filter to match Jinja2 behaviour.

Closes #282.

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 #1279 into master will decrease coverage by 0.03%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #1279      +/-   ##
==========================================
- Coverage   89.60%   89.57%   -0.04%     
==========================================
  Files          22       22              
  Lines        3020     3030      +10     
==========================================
+ Hits         2706     2714       +8     
- Misses        314      316       +2     

Impacted Files	Coverage Δ
nunjucks/src/node-loaders.js	65.67% <0.00%> (-2.99%)	⬇️
nunjucks/src/filters.js	96.24% <100.00%> (+0.13%)	⬆️

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[ogonkov]

It's better to merge it after #1278 since they almost the same.

[ogonkov]

@fdintino now it's ready.

Codecov shows uncovered lines for file that is not in my PR for some reason

[ogonkov]

@fdintino Can you please trigger build in Travis for master it should fix coverage

[ogonkov]

@fdintino ?

[ogonkov]

@fdintino can we finish this Pull Request? :)

[fdintino]

@ogonkov : absolutely! My apologies though: it may not be for a week or two, at least. Only just got back to work from paternity leave, still managing remote work with a newborn and getting through a backlog of open source project maintenance.