Allow disabling of warnings in config.ml

[hannesm]

As suggested by @gasche in abbysmal/Canopy#66 -- in this specific case warning 42 is signalled due to Syndic exposing several constructors with the same name from a single module. There can be nothing done in Canopy to avoid this problem. It would be great to be able to specify in config.ml which warnings to suppress.

[hannesm]

Further note from @gasche in the Canopy issue, indicating that we should maybe not enable warning 42 at all:

My mental model is that few people would enable warning 42: it used used by either skeptics that don't believe in type-based disambiguation, or by people that want to guarantee that their code remains compatible with ocaml < 4.01. As a a library author, I would not accept making a breaking change to cater to the people using it.