Enable multi-line mode in GHCi

Docs: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/interactive-evaluation.html#ghci-multiline

You can also use `:{` and `:}` to begin and end multi-line input: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/interactive-evaluation.html#ghci-stmts