Improve camel case implementation

[stoewer]

Changes:

• Preserve case when converting from upper to lower camel case and vice versa
• Change linter to golangci-lint
• Switch from travis to circle-ci

[coveralls]

Coverage increased (+1.3%) to 100.0% when pulling 29ef824 on improve-camel-case into 56c6d9c on master.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@56c6d9c). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master     #5   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?      4           
  Lines             ?     71           
  Branches          ?      0           
=======================================
  Hits              ?     71           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
helper.go	100% <100%> (ø)
camel.go	100% <100%> (ø)

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