Add support for generics in extractor definition

[kibertoad]

Previously generics were ignored inside extractors that made using them in type-safe way a huge hassle.
This makes extractor definitions respect generics specified for the route.

[codecov-io]

Codecov Report

Merging #13 (144f61d) into master (8ca51f0) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #13   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           34        34           
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  Hits            34        34           

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[kibertoad]

@simoneb My apologies for a barrage of requests, but if you could release a new version whenever you have some free time, that would be appreciated! Sorry that new issues keep coming up, still fleshing out our pilot implementation.

[simoneb]

1.2.1 contains this change