MangaDex: Add "download all" option

[bake]

Hi, I've added the option to download all chapters of a manga from MangaDex. But before considering merging this, there are a few questions:

It currently switches between "download one chapter" and "download all chapters" based on the given URL. For example, if you pass mangadex.org/title/123, it'll download all and if you pass mangadex.org/chapter/123 it'll just download one chapter. How should it, for example, behave, if one doesn't set the -all flag but passes a manga URL?

Another problem are different languages: It uses the same flag as MangaRock ( -country), but expects "language codes" like gb or sa instead of names. Should this be translated internally or would it be enought to note it in the readme?

Hope this helps #41.

[coveralls]

Pull Request Test Coverage Report for Build 219

• 7 of 24 (29.17%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.5%) to 79.104%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/sites/loader.go	0	1	0.0%
pkg/sites/mangadex.go	7	23	30.43%
<!--	Total:	7	24	29.17%	-->

Totals
Change from base Build 216:	-1.5%
Covered Lines:	636
Relevant Lines:	804

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💛 - Coveralls

[Church-]

@bake Great to see this, wanting to move off mangarock given they're planning to go to a fully legit platform in the near future.

[Girbons]

Hi @bake thanks for this PR!

if you pass mangadex.org/title/123, it'll download all and if you pass mangadex.org/chapter/123 it'll just download one chapter. How should it, for example, behave, if one doesn't set the -all flag but passes a manga URL?

By default if you pass a manga URL (mangadex.org/title/123) -all flag should be implicit because the user didn't specify a chapter.

Another problem are different languages: It uses the same flag as MangaRock ( -country), but expects "language codes" like gb or sa instead of names. Should this be translated internally or would it be enought to note it in the readme?

I think that we can note it in the readme for now.