Add initializer and initargs

[gwerbin]

Here is an initial draft implementation of https://github.com/nalepae/pandarallel/issues/231.

Still to-do:

• [ ] New documentation
• [ ] Test cases
• [ ] Type hints

I also see a lot of missing argument and return type hints across the codebase. I can add some of those while I am working on this PR, if that's OK.

[codecov[bot]]

Codecov Report

Merging #232 (e745151) into master (b78c63c) will decrease coverage by 4.66%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
- Coverage   91.37%   86.72%   -4.66%     
==========================================
  Files          12       12              
  Lines         580      580              
==========================================
- Hits          530      503      -27     
- Misses         50       77      +27     

Impacted Files	Coverage Δ
pandarallel/core.py	91.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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