Added Viper configuration (#31)

[vmarchese]

I have made the following changes to Close #31 :

• Added viper support with the following ways in order of precedence:
  • command line flags
  • environment variables
  • configuration files
• The error in the viper.BindPFlag now bubbles up to the main so I had to change the new<command> signature
• Added a doc.go file in the root folder to have a general documentation in the godoc
• Added a generic test in cmd/gremlins_test.go to check the correct order of precedence for cfg files and env vars
• updated docs to reflect the changes

The configuration file is a YAML file structured as follows:

<command>:
   <flag name>: <flag value>
   <flag name>: <flag value>
   ...

For the current command (i.e. unleash) is trivially:

unleash:
  dry-run: true
  tags: tag1,tag2,tag3

[codecov]

Codecov Report

Merging #75 (32a3427) into main (d86f93f) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   86.38%   86.77%   +0.39%     
==========================================
  Files          10       11       +1     
  Lines         573      590      +17     
==========================================
+ Hits          495      512      +17     
  Misses         61       61              
  Partials       17       17              

Flag	Coverage Δ
unittests	86.77% <100.00%> (+0.39%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/configuration/viper.go	100.00% <100.00%> (ø)

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