Allow user configurable command line options for build tools

There is currently no mechanism to control how tools like `happy` are invoked by cabal. Other tools that would be nice to control over include (but not limited to), `hsc2hs`, `c2hs`, and `alex`.

A simple option would be to add a field `happy-options:` to the cabal file format. An even better solution would be for the mechanism to allow the options to be per module. An even better solution would make the addition of tools with options into a generic configurable option so that new build tools can be added without modifying cabal.

Start by reviewing how Cabal currently invokes build tools such as happy, hsc2hs, c2hs, and alex, and how options are represented in the cabal file format. Compare the proposed tool-specific, per-module, and generic configuration approaches. Done should be a decided configuration design that supports the required tools and clearly defines its scope.

Written by the indexing model from the issue text.