Optionally add the revison number to Paths_$pkgid.version

It'd be nice if `cabal build` could automatically add the revision number for the appropriate SCM to the `versionTags` field of `Paths_$pkgid.version`. I envision the following usage scenario:

```
$ cabal configure --revision-number # or --build-id, or whatever
$ cabal run -- --version
1.0-0a4f3c6g
$ cabal configure
$ cabal run -- --version
1.0
```

For extracting the revision number from a working copy there is the [vcs-revision](http://hackage.haskell.org/package/vcs-revision) package on Hackage (only SVN, Git and Mercurial support ATM).

Start by tracing how cabal configure and cabal build generate Paths_$pkgid.version and populate its versionTags field. Review the vcs-revision package and its supported SCMs, then define the opt-in behavior described in the issue: include a revision in --version when requested, while the default remains 1.0.

Written by the indexing model from the issue text.