Add support for Slovak locale

[matus-cuper]

I added support for Slovak language, mainly by copying and rewriting Polish and Sweden locale code.

[coveralls]

Pull Request Test Coverage Report for Build 584

• 169 of 176 (96.02%) changed or added relevant lines in 12 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 94.84%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/devskiller/jfairy/producer/person/locale/sk/SkNationalIdentificationNumberProvider.java	41	44	93.18%
src/main/java/com/devskiller/jfairy/producer/company/locale/sk/SkVATIdentificationNumberProvider.java	25	29	86.21%

Totals
Change from base Build 580:	0.1%
Covered Lines:	1507
Relevant Lines:	1589

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💛 - Coveralls