Reader Monad for Flags

cabal is designed mostly without the use of monads. This leads to functions passing context via arguments along. And even so explicitly.

It seems like there is some interest in having cabal be partially refactored to use a Reader Monad for Flags (global, command specific, and **verbosity**).

I believe carrying a Record along as `context` instead of passing arguments explicitly might improve the situation, however a Monadic approach would even free one of having to pass the context explicitly.

The questions that remains are:
- 👍 or 👎
- How should the monad stack look? (I'd prefer not to hand roll it)
- What monad libraries can we use for convenience? (boot library restrictions)