Add MangaDex site engine

[Techwolfy]

No description provided.

[codecov]

Codecov Report

Merging #159 into master will decrease coverage by 0.38%.
The diff coverage is 35.29%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   82.65%   82.27%   -0.39%     
==========================================
  Files          72       73       +1     
  Lines        6280     6331      +51     
  Branches      423      431       +8     
==========================================
+ Hits         5191     5209      +18     
- Misses        974     1007      +33     
  Partials      115      115              

Impacted Files	Coverage Δ
dosagelib/plugins/mangadex.py	35.29% <35.29%> (ø)

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[TobiX]

Looks fine until someone implements #148 ;)