Added linefeed

Plugins that perform operations that depend on newlines should use the correct linefeed characters for the platform they're run on.

This PR provides `util.linefeed`, which is determined based on `os.platform()`. The [method used is nearly identical to the method used by grunt-util](https://github.com/gruntjs/grunt-util/blob/d35eb446964b7e62f81be28cfe047968c5416e91/lib/util/strings.js#L15), except that it uses `os.EOF` when it's available and `os.platform()` instead of `process.platform` as the `os` module is already available.