Add a default keyword argument to dumps

[garenchan]

dump and dumps functions in python json stdlib have a default keyword argument.
It's useful for serializing complex objects. So I think supporting this argument will improve
compatibility and flexibility of ujson.

https://docs.python.org/3/library/json.html

If specified, default should be a function that gets called for objects that can’t otherwise be serialized. It should return a JSON encodable version of the object or raise a TypeError. If not specified, TypeError is raised.

This should resolve #436 .

[codecov-commenter]

Codecov Report

Merging #470 (f7d9ec1) into main (8f5ad61) will increase coverage by 0.29%.
The diff coverage is 98.11%.

❗ Current head f7d9ec1 differs from pull request most recent head b366434. Consider uploading reports for the commit b366434 to get more accurate results

@@            Coverage Diff             @@
##             main     #470      +/-   ##
==========================================
+ Coverage   88.65%   88.94%   +0.29%     
==========================================
  Files           6        6              
  Lines        1631     1683      +52     
==========================================
+ Hits         1446     1497      +51     
- Misses        185      186       +1     

Impacted Files	Coverage Δ
python/objToJSON.c	84.52% <96.29%> (+0.76%)	⬆️
tests/test_ujson.py	99.54% <100.00%> (+0.02%)	⬆️

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[hugovk]

Thanks!