implement project builder

[martinohmann]

This refactors the project creation logic to use the builder pattern because it allows for more flexibility when dealing with lots of arguments where most of them are optional.

[codecov-io]

Codecov Report

Merging #55 into master will increase coverage by 1.80%.
The diff coverage is 92.45%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   73.67%   75.48%   +1.80%     
==========================================
  Files          25       25              
  Lines        1155     1175      +20     
==========================================
+ Hits          851      887      +36     
+ Misses        214      209       -5     
+ Partials       90       79      -11     

Impacted Files	Coverage Δ
internal/skeleton/skeleton.go	65.11% <0.00%> (-4.89%)	⬇️
internal/project/builder.go	95.48% <95.48%> (ø)
internal/project/file.go	100.00% <100.00%> (ø)
internal/template/render.go	80.00% <100.00%> (+7.27%)	⬆️

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