Added more flexbility for enable/disable per-page

We are using your plugin on our `learn.getgrav.org` site and noticed it was causing problems on some pages. We started digging into your plugin a little and noticed that it processed on every page even though we only needed it on one.  I have added the ability to set `all_pages: false` and then just enable on pages with `videoembed: true` or `videoembed: { .. options .. }`.  Also if you set `all_pages: true` you can disable on a specific page by setting `videoembed: false`.  

I have not dug further to find out what exactly was causing the issue, but just having these options makes it perform better and with more control than before.