Support finally tasks

[evidolob]

Fix: #345

Demo:

[codecov-commenter]

Codecov Report

Merging #401 into master will increase coverage by 0.18%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   74.51%   74.70%   +0.18%     
==========================================
  Files          58       58              
  Lines        3834     3854      +20     
  Branches      693      699       +6     
==========================================
+ Hits         2857     2879      +22     
+ Misses        977      975       -2     

Impacted Files	Coverage Δ
src/yaml-support/tkn-yaml.ts	89.53% <96.29%> (+0.47%)	⬆️
src/model/pipeline/pipeline-model.ts	80.65% <100.00%> (+1.17%)	⬆️
src/pipeline/pipeline-graph.ts	76.10% <100.00%> (ø)

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[evidolob]

@siamaksade I'm not sure about final task in graph