feat: add support for command file extensions

[devshawn]

Adds support for file extensions such as sh and bat on commands such as kafka-topics.

Related:

• Issue Add support for .sh kafka commands in the PATH #18
• Issue Add additional docs for working with kafka in docker #15

[codecov-io]

Codecov Report

Merging #19 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   96.79%   96.86%   +0.07%     
==========================================
  Files          12       12              
  Lines         562      575      +13     
==========================================
+ Hits          544      557      +13     
  Misses         18       18

Impacted Files	Coverage Δ
kafkashell/executor.py	100% <100%> (ø)	⬆️

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