Enhancement: Implement IndentNormalizer

[localheinz]

This PR

• requires localheinz/json-printer
• implements an IndentNormalizer
• requires phpspec/prophecy:^1.7.1 (see Allow PHP5 keywords methods generation on PHP7. phpspec/prophecy#337)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@5d8b3e2). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master     #1   +/-   ##
=======================================
  Coverage          ?   100%           
  Complexity        ?      6           
=======================================
  Files             ?      1           
  Lines             ?     15           
  Branches          ?      0           
=======================================
  Hits              ?     15           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ	Complexity Δ
src/IndentNormalizer.php	100% <100%> (ø)	6 <6> (?)

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