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]

Codecov Report

Merging #91 (2bf9b27) into master (aafcf7c) will not change coverage.
The diff coverage is 100.00%.

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

Impacted Files	Coverage Δ
src/colord.ts	100.00% <100.00%> (ø)
src/manipulate/lighten.ts	100.00% <100.00%> (ø)

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