Add option to ignore pages that don't contain the flattr meta tag

[genseirin]

Currently, when I enable flattring for a domain, all pages on that domain are being flattred. I would like to suggest an option in the extension so that web pages are only flattred if they contain the meta tag.

Benefits:

1. Creators could fine-tune which content can be flattred by omitting the Flattr meta tag <meta name="flattr:id" content="foo"> on some pages. (For example on sign-in pages.)

2. For contributors this options would help remove the clutter on their dashboards since they only see pages where their flattr actually counts.

3. This would also add an extra layer of opt-in privacy because not all URLs are sent over to the server.