Add support for readcomiconline.to

[jkernech]

Thank you for open-sourcing this project @Girbons!

This PR add support for a new Comic website: https://readcomiconline.to

I found that readcomiconline.to is more reliable especially for older comics, sometimes comicextra.com chapters are missing some images.

[coveralls]

Pull Request Test Coverage Report for Build 291

• 71 of 82 (86.59%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 78.372%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/sites/loader.go	0	2	0.0%
pkg/sites/readcomiconline.go	71	80	88.75%
<!--	Total:	71	82	86.59%	-->

Totals
Change from base Build 289:	0.9%
Covered Lines:	645
Relevant Lines:	823

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💛 - Coveralls