Replace reqwest with ureq

[jwodder]

Closes #71.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 132 lines in your changes missing coverage. Please review.

Project coverage is 83.18%. Comparing base (80c44c3) to head (6cbad2d).

Files	Patch %	Lines
src/client.rs	0.00%	57 Missing ⚠️
src/client/util.rs	0.00%	56 Missing ⚠️
src/main.rs	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   82.63%   83.18%   +0.55%     
==========================================
  Files           7        7              
  Lines        3173     3152      -21     
==========================================
  Hits         2622     2622              
+ Misses        551      530      -21     

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