Add a command that creates a skeleton script

For writing scripts, it would be nice if stack could generate a basic script skeleton. I'd imagine the following behaviour:

```
$ stack new-script turtle.hs
$ cat turtle.hs
#!/usr/bin/env stack
{- stack
--resolver lts-7.2
--install-ghc
runghc
--package base
--package turtle
--
-hide-all-packages
-}

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = echo "Hello World!"
```

The command should pick up `--resolver` and possibly other options similarly to `stack new`.