Add a basic codecov config to show commit status

[elia]

Summary

Fixes #211

Use coverage to understand what major areas or edge cases should be tested.

Correctly translates paths from the dummy-app to the templates dir, so that sources are visible in the CodeCov UI.

ERB look off from time to time, but it's not clear if a line is lost in translation or it's a ruby+ERB bug.

Checklist

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[codecov[bot]]

Codecov Report

Merging #204 (4b0077f) into master (d74d901) will decrease coverage by 1.26%. The diff coverage is 100.00%.

:exclamation: Current head 4b0077f differs from pull request most recent head bca8001. Consider uploading reports for the commit bca8001 to get more accurate results

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   94.16%   92.91%   -1.26%     
==========================================
  Files          15      124     +109     
  Lines         360     1580    +1220     
==========================================
+ Hits          339     1468    +1129     
- Misses         21      112      +91     

Impacted Files	Coverage Δ
Rakefile	100.00% <100.00%> (ø)
config/routes.rb	100.00% <100.00%> (ø)

... and 125 files with indirect coverage changes

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