new: added 'path' config option to limit commit messages

[dfugate]

This introduces a new optional 'path' config file parameter which allows end-users to run 'git log ... [path]' instead of 'git log ...'. The net effect here is this makes it easy to limit commits to those that affected a certain set directory and its contents only. Hopefully this capability didn't already exist in some other capacity:)

[codecov[bot]]

Codecov Report

Merging #112 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #112   +/-   ##
=======================================
  Coverage   88.45%   88.45%           
=======================================
  Files           2        2           
  Lines         840      840           
  Branches      168      168           
=======================================
  Hits          743      743           
  Misses         58       58           
  Partials       39       39

Impacted Files	Coverage Δ
src/gitchangelog/gitchangelog.py	88.34% <100%> (ø)	:arrow_up:

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