Add -l/--loglevel option

[blueyed]

Ref: #25

[codecov]

Codecov Report

Merging #26 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   96.02%   96.14%   +0.12%     
==========================================
  Files          15       15              
  Lines        1332     1375      +43     
  Branches      157      160       +3     
==========================================
+ Hits         1279     1322      +43     
  Misses         35       35              
  Partials       18       18

Flag	Coverage Δ
#py27	95.85% <100%> (+0.13%)	⬆️
#py34	94.32% <100%> (+0.18%)	⬆️
#py35	95.26% <100%> (+0.15%)	⬆️
#py36	95.26% <100%> (+0.15%)	⬆️

Impacted Files	Coverage Δ
tests/test_main.py	100% <100%> (ø)	⬆️
covimerage/cli.py	94.21% <100%> (+0.3%)	⬆️
tests/test_logging.py	100% <100%> (ø)	⬆️

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