Allow for swappable SiteDefaults implementation

[jesseleite]

With this PR, you can now override where site defaults are stored by providing a custom SiteDefaults implementation:

$this->app->bind(\Statamic\SeoPro\SiteDefaults::class, \App\MySiteDefaults::class);

This is for @ryanmitchell to store site defaults in a database by overloading the save and getDefaults methods ❤️

Closes #298 (This PR is just a slightly simpler alternative)

[jesseleite]

Thoughts on this @ryanmitchell? Do you see any downsides to this approach?

Also curious if you have need to override the blueprint() method? If so, I could make this method non-static / an overridable instance method 👍

[ryanmitchell]

Definitely simpler, i should have read the code more fully and done that :)

We don't need to override the blueprint method but it may be a good idea to change it for others.

[jesseleite]

Re: Blueprint method... Agreed and done 🤘