* New version_semantics value: git-describe

Designed for use with version numbers produced by "git describe"
command.  Presense of the "offset" part indicates an alpha release,
its absense indicates a stable one:

   0.9-14-gc4f98f0	alpha
   1.0.2		stable

* New configuration stagement: version_extract

Defines a sed expression to use for extracting version number from the
name of an archive.