Handle merge references correctly

[otsenkun]

fixes #59

[otsenkun]

Hi,

Thanks for the work you've done already :)
Let me know if I missed anything on this pull request

[otsenkun]

Hi @andrewlock, I was wondering if you can take a look at this anytime soon :)

[andrewlock]

Hi @otsenkun, thanks for this, merge keys aren't something I've ever run into! Unfortunately, this will technically be a breaking change, so I think we may need to put the behaviour behind a feature flag.

Also, would it be possible to add some additional tests for basic Anchors and Aliases (without merge keys), just to confirm we haven't changed existing behaviour?

Thanks!