coffee preprocessor supports squiggly arrows (~>)

In CoffeeScript code, indented blocks after a squiggly arrow will be
compiled in-place as APL:

    f = ~>
        a ← 2
        b ← 3
        ⌽⍳a+b

    console.info f()   # prints [ 4, 3, 2, 1, 0 ]

(issue #4)