Add issue-number-only option

[Jan0660]

Adds an option to save as issue number only instead of chapter name + issue number, e.g. 2.cbz instead of A Case-2.cbz

[coveralls]

Pull Request Test Coverage Report for Build 93

• 7 of 8 (87.5%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 58.746%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/core/core.go	2	3	66.67%
<!--	Total:	7	8	87.5%	-->

Totals
Change from base Build 89:	0.08%
Covered Lines:	628
Relevant Lines:	1069

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💛 - Coveralls

[Girbons]

Thank you @Jan0660!

I'm going to merge and release ASAP