Update .gitignore file for convenient use with Xcode.

[eonil]

Hello~

Xcode constantly makes changes in xcuserdata, and Finder also make changes in .DS_Store files.
I believe this addition to gitignore can help many people' life easier.

[codecov-io]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          34       34           
  Lines        9079     9079           
=======================================
  Hits         8968     8968           
  Misses        111      111

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[lorentey]

Thanks!