review allow_playback_repeats configuration option

[sckott]

my hunch is this is not used correctly right now

It's used in the HTTPInteractionList class in the method matching_used_interaction_for at https://github.com/ropensci/vcr/blob/master/R/http_interaction_list.R#L225 & usage in ruby vcr https://github.com/vcr/vcr/blob/master/lib/vcr/cassette/http_interaction_list.rb#L91-L94

demonstration of how it should work in Ruby vcr https://relishapp.com/vcr/vcr/v/5-1-0/docs/request-matching/playback-repeats

note: this is not a configuration option but meant to be used when calling use_cassette/insert_cassette