Truncate the displayed source code when it exceeds 6 lines - #589

Related to #573

Example output:

```
dhall <<< './test.dhall'

Use "dhall --explain" for detailed errors

↳ ./test.dhall

Error: You can only append ❰List❱s with matching element types

- Natural
+ Bool

            [ 1
            , 1
            , 1
✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈✂┈┈┈┈┈┈┈
            , 1
            ] # [ True ]

/Users/gabriel/proj/dhall/test.dhall:1:13
```