Default value on null, undefined or wrong type.

Hi,

I have several use cases where it makes sense to fall back to a default value when the key you're looking for is either null, missing entirely or of the wrong type.

The following pattern is described in the documentation:
```haskell
v1 <- o .:? "optFieldWithDefault" .!= 1.0 -- fails if optFieldWithDefault is set to a boolean for instance
```

This works fine for `null` or missing keys, but fails when the key is of the wrong type.
I ended up defining my own operator:
```haskell
(.|>) :: Parser (Maybe a) -> a -> Parser a
(.|>) parser def = parser .!= def <|> pure def
```

Which is used the exact same way as `.!=` but also falls back to your desired default value on invalid type:
```haskell
v1 <- o .:? "optFieldWithDefault" .|> 1.0 -- will default to 1.0 on null, undefined, or unexpected type
```

Would it make sense to add such an operator to Aeson, possibly under a different name ?