Interfaces and inheritance support

[MichalLytek]

Closes #6 🎉

[codecov]

Codecov Report

Merging #35 into master will increase coverage by <.01%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   92.53%   92.53%   +<.01%     
==========================================
  Files          36       38       +2     
  Lines         442      536      +94     
  Branches       81       96      +15     
==========================================
+ Hits          409      496      +87     
- Misses         32       39       +7     
  Partials        1        1

Impacted Files	Coverage Δ
src/metadata/metadata-storage.ts	100% <100%> (ø)	⬆️
src/decorators/GraphQLInterfaceType.ts	100% <100%> (ø)
src/utils/buildSchema.ts	100% <100%> (ø)	⬆️
src/decorators/GraphQLObjectType.ts	100% <100%> (ø)	⬆️
src/schema/GeneratingSchemaError.ts	100% <100%> (ø)
src/decorators/index.ts	100% <100%> (ø)	⬆️
src/helpers/decorators.ts	83.33% <66.66%> (ø)	⬆️
src/schema/schema-generator.ts	94.57% <94.62%> (-1.43%)	⬇️
src/decorators/Authorized.ts	0% <0%> (-50%)	⬇️
src/decorators/Subscription.ts	0% <0%> (-50%)	⬇️
... and 2 more

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