Add support for circular structures

[denizdogan]

This library does not support circular references, but if we could still get the same performance while supporting them it would be great! An alternative would be to add an option to enable it, so that existing users won't be affected and still get the same super-fast cloning.

[codeandcats]

Thanks for the suggestion. This would be a great feature.

I suspect it would incur a non trivial performance hit, but as you say could be an 'opt-in' feature via options. I should have some time over the holiday break to try this out.

[jasonkoon]

An ability to work for circular references would be nice. Or the ability to prune a circular reference?