Add support for MERGE_READY_STATE option

[BigGillyStyle]

This is based on the discussion in #203 and in particular this comment

[SaraRavasio]

nice one! but what if the MERGE_READY_STATE always has those values plus the one that can be passed? so it's not the default ones or the other but all of them.
would that make sense?
cc @pascalgn

[BigGillyStyle]

what if the MERGE_READY_STATE always has those values plus the one that can be passed?

That's a bit of an API preference that I'll defer to @pascalgn Would it be better to have the items provided by MERGE_READY_STATE to be merged with the default or replace the default state entirely (as was done originally in this PR)? The downside of what you are suggesting is that you couldn't override the defaults. However, the upside is that you don't have to re-state (or necessarily even know about) the default values. I could see either case.

[pascalgn]

Thanks for the PR, it's merged now! 👍