Config file as an option and print help if invalid argument

[andrewdavidmackenzie]

Fixes #779

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 28%. Comparing base (9856979) to head (110547d).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/piggui.rs	0%	13 Missing ⚠️
src/piglet.rs	0%	6 Missing ⚠️
src/persistence.rs	0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #781   +/-   ##
=====================================
- Coverage      28%    28%   -0%     
=====================================
  Files          46     46           
  Lines        5958   5963    +5     
=====================================
  Hits         1640   1640           
- Misses       4318   4323    +5     

Flag	Coverage Δ
unittests	28% <0%> (-<1%)	⬇️

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