refactor(viper): add viper to handle multi format config

[wass3r]

closes #15

❤ THANKS FOR HELPING OUT :D

Proposed change

Integrates spf13/viper to be able to handle multiple config formats

Types of changes

What types of changes is this pull request introducing to flottbot? Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

You can fill this out after creating your PR. Put an x in the boxes that apply

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Pull Request Test Coverage Report for Build 123

• 3 of 36 (8.33%) changed or added relevant lines in 5 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 38.078%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/rules.go	0	11	0.0%
cmd/flottbot/main.go	0	22	0.0%

Files with Coverage Reduction	New Missed Lines	%
cmd/flottbot/main.go	4	9.84%

Totals
Change from base Build 111:	-0.07%
Covered Lines:	741
Relevant Lines:	1946

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💛 - Coveralls