feat: support grouping by date

[aeneasr]

What does this do / why do we need it?

Grouping by .Author.Date is currently not possible because time.Time is not string

How this PR fixes the problem?

Adding a type switch allowing users to use Author.Date in the commit group commit_groups.group_by

What should your reviewer look out for in this PR?

Using time.Time currently defaults to the date. Itt might be desirable (in the future) to make this configurable, but in the interest of simplicity I chose to use the internationally acceptable date format YYYY-MM-DD.

Check lists

• Test passed
• Coding style (indentation, etc)

[coveralls]

Pull Request Test Coverage Report for Build 4143270565

• 8 of 16 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 76.541%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
commit_extractor.go	8	16	50.0%

Totals
Change from base Build 4124882195:	-0.2%
Covered Lines:	1850
Relevant Lines:	2417

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💛 - Coveralls

[mavogel]

hi @aeneasr , thanks for contributing. Would you mind, merging / rebasing master and tackle the coverage? Thank you