add cubic curves

[LinuxUserJTB]

Bézier curves are part of most graphics/rendering interfaces, so they should be supported by Graphics.jl.
This adds Cairo.jl's curve_to and rel_curve_to to the path interface. Cairo.jl would then need to be adjusted to import these.

[codecov]

Codecov Report

Base: 42.64% // Head: 45.07% // Increases project coverage by +2.42% 🎉

Coverage data is based on head (f19ed44) compared to base (6623e27).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   42.64%   45.07%   +2.42%     
==========================================
  Files           1        1              
  Lines         136      142       +6     
==========================================
+ Hits           58       64       +6     
  Misses         78       78              

Impacted Files	Coverage Δ
src/Graphics.jl	45.07% <ø> (+2.42%)	⬆️

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