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 `process.platform`. The [method used is exactly the same as grunt-util](https://github.com/gruntjs/grunt-util/blob/d35eb446964b7e62f81be28cfe047968c5416e91/lib/util/strings.js#L15).