Fix declare class with qualified type identifier

[danez]

This makes declare class extends behave the same way as in flow.
The ast-token after the extends keyword, might be either Identifier or QualifiedTypeIdentifier.

declare class a extends b.c.d {}

To do that this change splits the parseGenericType into two functions, one for parsing genericType and on for qualifiedTypeIdentifier

Fixes #87

[codecov-io]

Current coverage is 96.92% (diff: 100%)

Merging #97 into master will increase coverage by <.01%

@@             master        #97   diff @@
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+ Hits           2832       2837     +5   
  Misses           90         90          
  Partials          0          0          

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