Allow multiple layout directories to be configured

I'm currently using a setup where multiple Jekyll blogs share (mostly) the same layout, so I've created a submodule that holds all layouts, plugins and assets and then I symlink them into the actual Jekyll blog, e.g.

    blog/
        _layouts/
            default.html -> ../_style/_layouts/default.html
        _style/
            _layouts/
                default.html

The same goes for plugins, too. For plugins, it is possible to simply pass an array to the `plugins` configuration directive, making it load plugins from several directories. This PR implements the same behavior for layouts as well.