Can't build due to type error. (Vue CLI)

[lukewhrit]

There's an error logged by Vue when building in production mode that causes the application to exit and not build. This error only occurs when building in production.

There's no return value for the customResultHandler field in the VueAxeOptions interface. This is required by tsc.

Since this fix is so easy I'll put a PR to fix it.

[ktquez]

Thanks @lukewhrit