[web3@1.0.0] Generated typings don't have BigNumber as a type

With `web3@1.0.0` as target, generated types no longer support `BigNumber` for the contracts. As an example, this is the typing generated with `web3@0.26.0` -- https://github.com/MARKETProtocol/types/blob/develop/types/MarketCollateralPool.ts#L178 and the below is generated using `web3@1.0.0` target which has `string` instead of `BigNumber` -- https://github.com/MARKETProtocol/types/blob/feature/upgrade-web3-1.0.0/types/MarketCollateralPool.d.ts#L44

`BigNumber` might need some additional parsing while generating the types I guess. Would be really great if this can be supported soon. Thanks :)