Add overwrite property

[nordsimon]

Hello

We are working on an isomorphic react app and i liked the way this package worked. However we have some metatags only used for specific pages, it felt a bit weird to duplicate those on all other pages just to make them disappear. It will also be a place for future bugs

This PR adds an overwrite property on the component. If it is set, all previous added meta tags will be removed when calling this block again. This way you will get a full reset and it will also align with how the server render works.

[casertap]

Hey @nordsimon , thanks for sharing. This is an interesting feature.
I guess you use it by doing overwrite=true in the props passed to the MetaTags component?
I do not fully understand yet, Can you please, provide an example describing the routes to your pages and the meta tags overwrite that is happening.

Thanks a lot.