Ignore whitespace only lines

As in Xcode's option: "Automatically trim trailing whitespace" > "Including whitespace-only lines"

Xcode's default value of these settings are following:

    "Automatically trim trailing whitespace" = YES
    "Including whitespace-only lines" = NO

Adding this feature will make Atom to be more friendly for users who are using Xcode by default settings.

I wrote this for the issue #13