Config update + extras

Hi Bart,

I noticed that by default this module only applies to pages using the generic 'Page' class. I think that most of the time you will want the SEO tab to be available on custom page types, and it will be a rare few pages where you will want to hide it. With this PR I've changed the configuration so that you only have to specify which pages will not include the SEO tab, otherwise it will be included by default. Makes it quicker to get up and running out of the box and means you don't have to add an entry to the SEO config each time you add a new page type to a project.

I also reinstated the google suggest filed in this PR and fixed a little issue where the preview wouldn't always load.

The config updates aren't backwards compatible so I guess if you accept this PR it would justify a major release number if you're following [semver](http://semver.org/). I didn't update the ReadMe.md to go with the changes as I wasn't sure you'd approve of them - but let me know if you want me to do that.