[EXPERIMENTAL] Add transfer nodes

[feluelle]

Resolves #15

[codecov-commenter]

Codecov Report

Merging #21 (4a6866b) into master (6d6e2ed) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          291       324   +33     
=========================================
+ Hits           291       324   +33     

Impacted Files	Coverage Δ
airflow_diagrams/__init__.py	100.00% <100.00%> (ø)
airflow_diagrams/airflow.py	100.00% <100.00%> (ø)
airflow_diagrams/class_ref.py	100.00% <100.00%> (ø)
airflow_diagrams/cli.py	100.00% <100.00%> (ø)
airflow_diagrams/utils.py	100.00% <100.00%> (ø)

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