Add support for changelogs and insecure packages

[jayfk]

• closes Add an API option #231
• closes Show changes info #22
• closes Implement is_insecure #4

[codecov]

Codecov Report

Merging #236 into master will increase coverage by 0.01%.
The diff coverage is 93.47%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   97.13%   97.15%   +0.01%     
==========================================
  Files           9       10       +1     
  Lines         943      984      +41     
==========================================
+ Hits          916      956      +40     
- Misses         27       28       +1

Impacted Files	Coverage Δ
pyup/__init__.py	100% <100%> (ø)	⬆️
pyup/updates.py	100% <100%> (ø)	⬆️
pyup/settings.py	66.66% <66.66%> (ø)
pyup/requirements.py	97% <93.33%> (+0.42%)	⬆️

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