Cabal version: provide (document?) a way to denote it's a development version

# Problem

Currently, there is no way to denote a certain cabal version is in development.

# Motivation

Most cabal packages I've seen won't bump version until next release.

But imagine that the package in question talks to some service and reports its version,
if someone checkouts git master branch, they would report the same as version as previously released version.

# Solutions

a) use git revision

b) hash all sources

c) encode versioning (odd-even, suffix .999)

I don't want to go into PROS/CONS right now but plan to fill those in later on.