Send user-agent via CLI runner

[svtkachenko]

Closes #105

[codecov-io]

Codecov Report

Merging #130 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   99.28%   99.29%   +<.01%     
==========================================
  Files          14       14              
  Lines         561      565       +4     
  Branches       76       76              
==========================================
+ Hits          557      561       +4     
  Misses          2        2              
  Partials        2        2

Flag	Coverage Δ
#unittests	99.29% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/schemathesis/__init__.py	100% <100%> (ø)	⬆️
src/schemathesis/runner.py	100% <100%> (ø)	⬆️

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