feat: Add histstyle kwarg to plot.shape_hist

[matthewfeickert]

Resolves #25

Add histstyle kwarg to enable setting step as the histstyle for mplhep. Still default to fill as default style.

* Add histstyle kwarg to allow for 'step' style (lines)
* Add example of using 'step' style to example notebook

[codecov]

Codecov Report

Merging #26 (543b79b) into master (8822879) will decrease coverage by 0.79%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   29.37%   28.57%   -0.80%     
==========================================
  Files           7        7              
  Lines         143      147       +4     
  Branches       27       28       +1     
==========================================
  Hits           42       42              
- Misses        101      105       +4     

Flag	Coverage Δ
unittests	28.57% <0.00%> (-0.80%)	⬇️

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Impacted Files	Coverage Δ
src/heputils/plot.py	14.78% <0.00%> (-0.54%)	⬇️

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