Replace reqwest with ureq

[jwodder]

Closes #71.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 140 lines in your changes missing coverage. Please review.

Project coverage is 83.22%. Comparing base (85acc89) to head (d2d63d4).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/client/util.rs	0.00%	63 Missing ⚠️
src/client.rs	0.00%	58 Missing ⚠️
src/main.rs	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   82.79%   83.22%   +0.42%     
==========================================
  Files           7        7              
  Lines        3156     3129      -27     
==========================================
- Hits         2613     2604       -9     
+ Misses        543      525      -18     

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