Fix InterpretOptions to work inside Optional/List. Fixes #33

Consider the following Haskell program:

```
{-# LANGUAGE DeriveAnyClass    #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}

import Dhall hiding (auto)

import qualified Data.Text.Lazy

interpretOptions :: InterpretOptions
interpretOptions = defaultInterpretOptions
    { fieldModifier = Data.Text.Lazy.dropWhile (== '_') }

data GitRepo = GitRepo
    { _host :: Text
    , _repo :: Text
    } deriving (Generic, Interpret, Show)

data BoxConfig = BoxConfig
    { _userName        :: Text
    , _dotfilesRepo    :: Vector GitRepo
    } deriving (Generic, Interpret, Show)

main :: IO ()
main = do
    x <- Dhall.input (autoWith interpretOptions) "./config"
    print (x :: BoxConfig)
```

Before this change the above program attempts to decode a value of type:

```
{ userName : Text, dotfilesRepo : List { _host : Text, _repo : Text } }
```

... when it should be decoding a value of type:

```
{ userName : Text, dotfilesRepo : List { host : Text, repo : Text } }
```

This change ensures that `InterpretOptions` correctly propagate to elements of
`List` or `Optional` values

src/Dhall.hs

@@ -404,10 +404,10 @@ instance Interpret Text where
     autoWith _ = text
 
 instance Interpret a => Interpret (Maybe a) where
-    autoWith _ = maybe auto
+    autoWith opts = maybe (autoWith opts)
 
 instance Interpret a => Interpret (Vector a) where
-    autoWith _ = vector auto
+    autoWith opts = vector (autoWith opts)
 
 {-| Use the default options for interpreting a configuration file