Newline macros

[MasonProtter]

This is my current attempt at #436, sorry it took me a little while to post it @c42f.

Lots of things work, but the main problem currently is that my criterion for detecting cases like (@foo x) is too broad. Specifically, cases like this aren't doing what we want:

julia> """
       (@foo function bar()
           @baz x
           y
       end)
       """ |> Meta.parse
:(#= none:1 =# @foo function bar()
          #= none:1 =#
          #= none:2 =#
          #= none:2 =# @baz x y
      end)

Basically, it's detecting that (@foo is matching the desired pattern, but then its also recursively applying the new parsing rule to @baz x and grabbing the y incorrectly.

I haven't yet figured out the right way to fix this.