added option for imaginary plotting in plot_eigenvalues

[macquarrielucas]

I added a keyword argument to the plot_eigenvalue method to allow plotting for the imaginary part of the eigenvalues.

This is a small change but I found it convenient in my own work.

Please forgive me if I'm doing anything wrong. This is my first pull request ever and open for feedback!

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.88%. Comparing base (39af849) to head (f2b6d69).
Report is 34 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/PlotsExt/plot.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
- Coverage   88.95%   88.88%   -0.07%     
==========================================
  Files          66       65       -1     
  Lines        9648     9725      +77     
==========================================
+ Hits         8582     8644      +62     
- Misses       1066     1081      +15     

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[rveltz]

Seems OK to me if useful to you. Thank you