Add base arg to int filter

[ogonkov]

Summary

Proposed change:

Add base arg for int filter, to match Jinja2 behaviour.

Closes #1307.

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-commenter]

Codecov Report

Merging #1309 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1309   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files          22       22           
  Lines        3045     3046    +1     
=======================================
+ Hits         2731     2732    +1     
  Misses        314      314           

Impacted Files	Coverage Δ
nunjucks/src/filters.js	96.25% <100.00%> (+0.01%)	⬆️

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