Add support for initializing member variables

The implementation follow TS rules:
- when the memberVariables option is enabled, the constructor must call the super constructor,
- when the memberVariables option is enabled and a class has initialized instance variables, the call to the super constructor must be the first statement in the class own constructor,
- instance variables initializers are evaluated when the class is instantiated.
> only if options.memberVariables

if !options.memberVariables then initStatements = [] and no statements would be added. It would then work as expected.

I haven't added an explicit test to keep the code simpler. Let me know if you prefer the explicit version, I'll update.