Add tree command

[sbidoul]

No description provided.

[codecov]

Codecov Report

Merging #29 into master will decrease coverage by 0.68%.
The diff coverage is 89.36%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
- Coverage   94.34%   93.65%   -0.69%     
==========================================
  Files          13       14       +1     
  Lines         601      694      +93     
  Branches      111      127      +16     
==========================================
+ Hits          567      650      +83     
- Misses         22       28       +6     
- Partials       12       16       +4     

Impacted Files	Coverage Δ
src/pip_deepfreeze/pip.py	91.13% <ø> (ø)
src/pip_deepfreeze/installed_dist.py	90.24% <77.77%> (-9.76%)	⬇️
src/pip_deepfreeze/tree.py	91.66% <91.66%> (ø)
src/pip_deepfreeze/__main__.py	94.87% <100.00%> (+0.58%)	⬆️

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