feat: implement relative option for lighten/darken manipulation

[EricRovell]

#90 issue has a valid point: the relative lightness manipulation has a lot of sense.

At first I thought it should be the default behavior, still, as it may be a breaking change, I implemented the feature as an optional relative: Boolean parameter.

[codecov-commenter]

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (aafcf7c) to head (2bf9b27).
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Additional details and impacted files

@@            Coverage Diff            @@
##            master       #91   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           41        41           
  Lines          622       623    +1     
  Branches       115       142   +27     
=========================================
+ Hits           622       623    +1     

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