Handle TS namespaces correctly

[Swatinem]

See #8 (comment)

export declare function Alert(props: AlertProps): JSX.Element;
export declare namespace Alert {
    var defaultProps: {
        type: string;
    };
}

This is also related to #7

I just recently learned what typescript namespaces actually are and how to use them in declaration files. I think its a little more complicated to make them work correctly.

[Swatinem]

should be mostly fixed by 69b1198