Improve AppVeyor integration

[felixfbecker]

• Add caching
• Add coverage reporting
• Add badge (missing secret token, @beyang)

[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   49.68%   49.91%   +0.23%     
==========================================
  Files          12       12              
  Lines        1737     1737              
  Branches      289      289              
==========================================
+ Hits          863      867       +4     
+ Misses        769      767       -2     
+ Partials      105      103       -2

Impacted Files	Coverage Δ
src/project-manager.ts	82.88% <0%> (+0.26%)	✅
src/util.ts	73.07% <0%> (+2.3%)	✅

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