Allow to define custom differ object in matchers

[sponomarev]

The questions about custom differ already appeared in the past in #627, #97. I think it could be valuable to allow custom differ objects in matchers.

The proposed solution should be completely backward compatible. And, I hope, will provide a way to extend custom RSpec matchers libraries.

[pirj]

Would you be interested in taking another approach to the problem @sponomarev ?

[benoittgt]

I am interested to work on this if @sponomarev can not, after I finish the task on documentation and yard. https://github.com/rspec/rspec.github.io/pull/131

[sponomarev]

@pirj Yeah, for sure. I'll take care of it this week. @benoittgt thanks for the offer, I think I'll deal with it on my own =)

[pirj]

@sponomarev A friendly reminder

[benoittgt]

@sponomarev Little bump on this one 😊

[JonRowe]

I'm closing this as part of the monorepo migration, we are interested in a pluggable differ overall but this isn't the right approach.