Factor default CSS rules for foldgutter out to standalone foldgutter.css

Creating a CodeMirror editor with folding enabled, I failed to notice in the manual and in the sample code that CSS rules need to be applied to make the fold-gutter visible. This pull request moves the default styles, which were in-line in the demo code, out to a separate .css file to make them more reusable. It also updates the manual to make it clearer to the user that styles have to be applied.