Fix usage of ArgumentParser

[outloudvi]

fix #24

https://github.com/python/cpython/blob/460ee5b994335994d4b5186c08f44e775b3e55fa/Lib/argparse.py#L1865

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.43%. Comparing base (ae465e2) to head (175107e).

Files	Patch %	Lines
mw2fcitx/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   77.43%   77.43%           
=======================================
  Files          18       18           
  Lines         523      523           
  Branches       84       84           
=======================================
  Hits          405      405           
  Misses         94       94           
  Partials       24       24           

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