Add support for documentation for executables

Please add support for documentation. This is very important for packages that only contain executables. I propose that in addition to the Data-Files and Data-Dir, the Doc-Files and Doc-Dir fields be supported. The contents mentioned would be installed in the obvious, platform specific location. For example, on Linux, if the binaries are at ${prefix}/bin, then the documents would go at ${prefix}/share/<Package Name>. Also, please change the program that is generated with the platform specific data directory target to also inform programs as to where the documentation is installed. In my case, it's the module Paths_cpsa. My program, CPSA, prints the data directory with "cpsa -h" so people can find its PDF files. When and if this change is made, I would like it to be able to continue to report the location of its documentation.

Please feel free to look at

http://hackage.haskell.org/package/cpsa-2.3.1/cpsa.cabal

if you need to see what motivates this change request.