Add `warn.conflicts` argument to `cluster_library()`

[PhilipBerg]

In my humble opinion, I think it would be really nice if multidplyr::cluster_library had a similar functionality as library's warn.conflicts. In many cases, you know what libraries you are sending to the cluster, and you might not need the warnings. In particular, for package's that runs multidplyr it can be very useful to be able to avoid the warnings (without needing to wrap cluster_library in additional code).

[hadley]

Thanks for the suggestion! Will definitely consider it when I'm next working on multidplyr.