(#32) Support for commit-msg hook with msg-file flag

[llorllale]

PR for #32:

• Added optional --msg-file flag for analyzing commit message found in this file instead of commits in the repo
• Added instructions on how to configure a git hook

[codecov]

Codecov Report

Merging #37 into master will decrease coverage by 0.36%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
- Coverage      84%   83.63%   -0.37%     
==========================================
  Files           4        4              
  Lines         100      110      +10     
==========================================
+ Hits           84       92       +8     
- Misses          8        9       +1     
- Partials        8        9       +1

Impacted Files	Coverage Δ
internal/commits/commits.go	85.18% <80%> (-1.18%)	⬇️

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