Added unit test

[tleonhardt]

Added a unit test related to the is_text_file() method used by do_load()

[codecov]

Codecov Report

Merging #174 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #174      +/-   ##
=========================================
+ Coverage   96.21%   96.3%   +0.08%     
=========================================
  Files           1       1              
  Lines        1136    1136              
=========================================
+ Hits         1093    1094       +1     
+ Misses         43      42       -1

Impacted Files	Coverage Δ
cmd2.py	96.3% <0%> (+0.08%)	⬆️

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