here-docs for multiple lines

[ikngtty]

cf. #57

In Haskell, literal string syntax for multiple lines is cumbersome.

"case x of\n" <>
"  Foo -> 1\n" <>
"  Bar x -> 2"

Now, here-docs are introduced ( #57 ), but they are not suitable for multiple line strings because their indentation is not smart.

      it "data constructor" $
        parseExpr
        [rawS|case x of
  Foo -> 1
  Bar x -> 2|]
        `shouldParse`
        -- ...

This is unclear and ugly!

New here-docs like below is desired.

[multiline|case x of
          |  Foo -> 1
          |  Bar x -> 2
|]