Add a proper system for stack warnings suppression / collection / display

From the discussion [here](https://github.com/commercialhaskell/stack/issues/1705#issuecomment-180062695):

> It'd be nice is every warning message could be disabled (analogous to GHC's `-fno-warn-*` options), which would cover this particular issue as well. It'd also be nice if repeated warnings were automatically suppressed and they could be emitted together for a given package instead of being interspersed with other output.
>
> Another reason to have a system like this is so that they can be moved to the end of the output. Currently it's easy for them to get lost in the midst of output.
>
> We've also sometimes needed to restructure code to plumb warning / error info to the end of output. It'd be nicer to store away warnings / errors at the location that generates them.