add encodePretty'' variant that takes Value instead of ToJSON a

Pretty clear. Just some information about my workflow.

I have some data types that are not instances of `ToJSON`. So I am creating `Value` from them in outside of `ToJSON` class. So I need a function that can pretty print `Values` (well, actually this what `aeson-pretty` aims). 

Also I haven't created `Value -> ByteString` function that uses default config, because it's not so necessary. 

Or you can choose alternative names like:

```haskell
encodeValue :: Value -> ByteString
encodeValue' :: Config -> Value -> ByteString
```