To prevent overriding the users settings, look and see if the user wants to allow softTabs

Just because a file is a mix of tabs and spaces doesn't mean the user wants to use softTabs. As a result, text-buffer should first look at the editor.softTabs preferance before determining what to use to prevent overriding the end user. This addresses [Issue 11](https://github.com/atom/text-buffer/issues/11).