Add support generic bounds

[olegshtch]

Allows to use generate extends bound on generic types.

[vojtechhabarta]

@olegshtch thanks for this nice PR.
I would wait for more use cases or comments before merging it.

[olegshtch]

Looks like #380 isn't fixed with this PR.

[olegshtch]

Rebased. Removed test for #380.

[vojtechhabarta]

@olegshtch I pushed fix for #380 - c231299

[olegshtch]

@vojtechhabarta I suppose generic bounds wasn't part of this commit?

[vojtechhabarta]

No, I think it is separate thing.

[olegshtch]

Is something left to do in this PR?

[vojtechhabarta]

I am not sure if this feature has high enough value to justify the change in generated output.
Do you have any real example where this feature might be useful?
The feature itself is nice but I think it can potentially break some existing user code.

[olegshtch]

We use a narrowing of types in a production Java code so prefer to have it in a TypeScript. Something like Message<T extends Body> where Body have a some guarantied fields.

[olegshtch]

Maybe add a new option to enable it?

[vojtechhabarta]

In general I prefer not to add new parameters because it is becoming harder for users to go through all typescript-generator parameters.

Currently typescript-generator outputs Message<T> which looses extends Body but then this type is used somewhere with particular type argument, for example:

interface Response {
    message: Message<HumanBody>;
}
interface HumanBody extends Body {
}

So when there is a JSON with Response data it is correctly typed.

[olegshtch]

Rebased