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

[mgsloan]

From the discussion here:

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.

[mgsloan]

It'd also be nice to be able to accumulate non-fatal errors. In this case, stack should continue work, but at the end output the errors and exit with a nonzero code. (see #2065)

[mgsloan]

Another good warning case to present at the end of output is when the config file has unrecognized fields.