Use the correct line endings for the current platform

This package is quite handy, but I noticed that new posts are always created with Windows-style CRLF line endings, rather than ones that are native to the current platform. This uses Node's [`os.EOL`](http://nodejs.org/docs/v0.11.10/api/os.html#os_os_eol) to generate the new post template with the proper line endings.

By the way, while I was looking for the best way to do this, I noticed that `TextBuffer` in Atom core seems to always use `\n`. If you'd prefer being consistent with that instead it would be an even more trivial change - I could change it to a [CoffeeScript multi-line string literal](http://coffeescript.org/#strings) and not have to do the `.join` stuff.