Changed several methods to return invisible(self) to allow chaining

[jayqi]

Enables the code shown in this example.

[codecov-io]

Codecov Report

Merging #21 into master will decrease coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   82.88%   82.72%   -0.16%     
==========================================
  Files          14       14              
  Lines         625      625              
==========================================
- Hits          518      517       -1     
- Misses        107      108       +1

Impacted Files	Coverage Δ
R/workflow_dag.R	91.62% <100%> (-0.47%)	⬇️
R/module_package_function.R	84.44% <100%> (ø)	⬆️
R/module_custom_function.R	93.33% <100%> (ø)	⬆️

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[jayqi]

This contributes to #20

[jameslamb]

I just assigned @cwschultz88 to this PR