Need an option to cast string to int

Hi,

I upgrade aeson to version 2.0.3.0 and gogol-1.0 fails because data type has an int64 field, but value is `"0"`

```
inputStages :: (Core.Maybe [Core.Int64]),
} deriving (Core.Eq, Core.Show, Core.Generic)

```

No source file, test, or entry point is named. Start by reproducing the gogol-1.0 failure with aeson 2.0.3.0 and inspect existing decoding options; done means the requested option is implemented and its behavior is covered by tests.

Written by the indexing model from the issue text.