Support for multidimensional arrays

[rklfss]

This PR adds the support for multidimensional arrays.

Therefore the isArray flag was removed in favor of a new array type model.

This is necessary because setting the isArray flag of a type that is already an array - which in fact happened for multidimensional arrays - has no effect.

convertNode(context: Context, node: ts.ArrayTypeNode): Type {
        let result = this.owner.convertType(context, node.elementType);

        if (result) { // What if isArray is already true here?
            result.isArray = true;
        } else {
            result = new IntrinsicType('Array');
        }

        return result;
}

Please consider the corresponding PR for themes: TypeStrong/typedoc-default-themes#48

[blakeembrey]

This seems like a useful change to me. Does anyone else want to review it?

Should we merge and release the theme changes as a new minor first, then update this PR and release a minor of TypeDoc?

[rklfss]

Any reactions on this one? I'll be definitely happy to get a second and third opinion.

However you handle the two repositories, both changes are not compatible without each other. So the way you mentioned sounds like a proper way to go.